In anderson test, added support for right skewed gumbel distribution.
In anderson test, added support for right skewed gumbel distribution.

The default anderson tests with dist='gumbel' assumes a left skewed
gumbel distro, gumbel_l.

For compatibility with the distros available in scipy.stats, added
functionality for the user to test right skewed gumbel distributions,
gumbel_r.

If user input is dist='gumbel', then 'gumbel_l' is assumed, as before.
On the top of that, added possibility for user input dist='gumbel_r'.

closes gh-2592

Removed nested elif blocks.

josephcslater/scipy,Newman101/scipy,anielsen001/scipy,aarchiba/scipy,lhilt/scipy,mdhaber/scipy,rgommers/scipy,anielsen001/scipy,befelix/scipy,nonhermitian/scipy,aeklant/scipy,aeklant/scipy,jjhelmus/scipy,WarrenWeckesser/scipy,scipy/scipy,gertingold/scipy,apbard/scipy,dominicelse/scipy,gdooper/scipy,gdooper/scipy,e-q/scipy,pyramania/scipy,mikebenfield/scipy,gfyoung/scipy,sriki18/scipy,jakevdp/scipy,surhudm/scipy,woodscn/scipy,behzadnouri/scipy,haudren/scipy,behzadnouri/scipy,kalvdans/scipy,pyramania/scipy,anntzer/scipy,ilayn/scipy,matthewalbani/scipy,WarrenWeckesser/scipy,andyfaff/scipy,jjhelmus/scipy,vigna/scipy,tylerjereddy/scipy,befelix/scipy,endolith/scipy,Stefan-Endres/scipy,ilayn/scipy,perimosocordiae/scipy,person142/scipy,gdooper/scipy,Newman101/scipy,gertingold/scipy,jamestwebber/scipy,haudren/scipy,jamestwebber/scipy,matthewalbani/scipy,perimosocordiae/scipy,Eric89GXL/scipy,jakevdp/scipy,gdooper/scipy,arokem/scipy,ilayn/scipy,Eric89GXL/scipy,pbrod/scipy,tylerjereddy/scipy,pschella/scipy,mdhaber/scipy,tylerjereddy/scipy,dominicelse/scipy,ilayn/scipy,perimosocordiae/scipy,pbrod/scipy,zerothi/scipy,apbard/scipy,anntzer/scipy,Stefan-Endres/scipy,lhilt/scipy,behzadnouri/scipy,Newman101/scipy,bkendzior/scipy,matthew-brett/scipy,apbard/scipy,e-q/scipy,nmayorov/scipy,dominicelse/scipy,andyfaff/scipy,sriki18/scipy,bkendzior/scipy,scipy/scipy,mikebenfield/scipy,josephcslater/scipy,pizzathief/scipy,kalvdans/scipy,nmayorov/scipy,nmayorov/scipy,zerothi/scipy,maniteja123/scipy,gfyoung/scipy,Eric89GXL/scipy,jor-/scipy,haudren/scipy,sriki18/scipy,gdooper/scipy,matthew-brett/scipy,kleskjr/scipy,andyfaff/scipy,Newman101/scipy,jakevdp/scipy,jamestwebber/scipy,perimosocordiae/scipy,mikebenfield/scipy,arokem/scipy,Newman101/scipy,rgommers/scipy,maniteja123/scipy,Eric89GXL/scipy,lhilt/scipy,jamestwebber/scipy,surhudm/scipy,Eric89GXL/scipy,jakevdp/scipy,sriki18/scipy,WarrenWeckesser/scipy,endolith/scipy,endolith/scipy,nonhermitian/scipy,vigna/scipy,befelix/scipy,maniteja123/scipy,vigna/scipy,jjhelmus/scipy,kalvdans/scipy,jor-/scipy,anielsen001/scipy,gfyoung/scipy,zerothi/scipy,matthewalbani/scipy,tylerjereddy/scipy,person142/scipy,anntzer/scipy,jamestwebber/scipy,behzadnouri/scipy,kleskjr/scipy,mdhaber/scipy,bkendzior/scipy,Newman101/scipy,endolith/scipy,scipy/scipy,haudren/scipy,andyfaff/scipy,jjhelmus/scipy,Stefan-Endres/scipy,perimosocordiae/scipy,pschella/scipy,anntzer/scipy,person142/scipy,pizzathief/scipy,behzadnouri/scipy,pschella/scipy,maniteja123/scipy,maniteja123/scipy,woodscn/scipy,surhudm/scipy,matthew-brett/scipy,sriki18/scipy,andyfaff/scipy,kleskjr/scipy,dominicelse/scipy,grlee77/scipy,pizzathief/scipy,arokem/scipy,jor-/scipy,pbrod/scipy,haudren/scipy,anielsen001/scipy,jakevdp/scipy,vigna/scipy,ilayn/scipy,matthew-brett/scipy,rgommers/scipy,grlee77/scipy,Stefan-Endres/scipy,anielsen001/scipy,pyramania/scipy,nonhermitian/scipy,jjhelmus/scipy,mdhaber/scipy,surhudm/scipy,aeklant/scipy,pizzathief/scipy,anntzer/scipy,sriki18/scipy,e-q/scipy,gertingold/scipy,pbrod/scipy,gfyoung/scipy,befelix/scipy,kleskjr/scipy,grlee77/scipy,aeklant/scipy,pbrod/scipy,aeklant/scipy,Stefan-Endres/scipy,nmayorov/scipy,pschella/scipy,e-q/scipy,tylerjereddy/scipy,woodscn/scipy,pizzathief/scipy,nmayorov/scipy,surhudm/scipy,grlee77/scipy,scipy/scipy,jor-/scipy,gfyoung/scipy,matthewalbani/scipy,mdhaber/scipy,surhudm/scipy,josephcslater/scipy,rgommers/scipy,pbrod/scipy,bkendzior/scipy,Eric89GXL/scipy,apbard/scipy,e-q/scipy,zerothi/scipy,nonhermitian/scipy,arokem/scipy,endolith/scipy,grlee77/scipy,pschella/scipy,scipy/scipy,woodscn/scipy,kalvdans/scipy,mikebenfield/scipy,josephcslater/scipy,vigna/scipy,gertingold/scipy,scipy/scipy,woodscn/scipy,andyfaff/scipy,apbard/scipy,zerothi/scipy,arokem/scipy,WarrenWeckesser/scipy,person142/scipy,WarrenWeckesser/scipy,mikebenfield/scipy,person142/scipy,matthewalbani/scipy,josephcslater/scipy,bkendzior/scipy,aarchiba/scipy,dominicelse/scipy,matthew-brett/scipy,aarchiba/scipy,jor-/scipy,aarchiba/scipy,perimosocordiae/scipy,befelix/scipy,WarrenWeckesser/scipy,zerothi/scipy,rgommers/scipy,pyramania/scipy,nonhermitian/scipy,haudren/scipy,kleskjr/scipy,maniteja123/scipy,behzadnouri/scipy,anntzer/scipy,kleskjr/scipy,aarchiba/scipy,endolith/scipy,lhilt/scipy,anielsen001/scipy,pyramania/scipy,woodscn/scipy,gertingold/scipy,kalvdans/scipy,lhilt/scipy,ilayn/scipy,mdhaber/scipy,Stefan-Endres/scipy