BUG: stats: Fix spurious warnings in genextreme.
BUG: stats: Fix spurious warnings in genextreme.

Use _lazywhere to fix spurious warnings that can occur when using
stats.genextreme.  An example before this change is:

	In [3]: np.random.seed(123)

	In [4]: s = genextreme.rvs(-0.25, loc=2, scale=0.01, size=1000)

	In [5]: pg = genextreme.fit(s)
	/Users/warren/miniconda3scipy/lib/python3.5/site-packages/scipy-1.0.0.dev0+1e8cadd-py3.5-macosx-10.6-x86_64.egg/scipy/stats/_continuous_distns.py:1894: RuntimeWarning: invalid value encountered in true_divide
	  np.sign(c)*(-g3+(g2+2*g2mg12)*g1)/((g2mg12)**(3./2.)))
	/Users/warren/miniconda3scipy/lib/python3.5/site-packages/scipy-1.0.0.dev0+1e8cadd-py3.5-macosx-10.6-x86_64.egg/scipy/stats/_continuous_distns.py:1899: RuntimeWarning: invalid value encountered in true_divide
	  (g4+(-4*g3+3*(g2+g2mg12)*g1)*g1)/((g2mg12)**2))

	In [6]: pg
	Out[6]: (-0.24373841815709019, 2.0000458814008155, 0.0099947358516431356)

arokem/scipy,zerothi/scipy,andyfaff/scipy,gertingold/scipy,Eric89GXL/scipy,jor-/scipy,perimosocordiae/scipy,grlee77/scipy,Eric89GXL/scipy,gfyoung/scipy,jakevdp/scipy,gertingold/scipy,pizzathief/scipy,andyfaff/scipy,gfyoung/scipy,e-q/scipy,jamestwebber/scipy,arokem/scipy,e-q/scipy,pbrod/scipy,jakevdp/scipy,ilayn/scipy,jamestwebber/scipy,perimosocordiae/scipy,scipy/scipy,arokem/scipy,scipy/scipy,jor-/scipy,ilayn/scipy,endolith/scipy,zerothi/scipy,aeklant/scipy,person142/scipy,jor-/scipy,apbard/scipy,aeklant/scipy,mdhaber/scipy,josephcslater/scipy,gertingold/scipy,aeklant/scipy,nmayorov/scipy,ilayn/scipy,pbrod/scipy,pizzathief/scipy,zerothi/scipy,pizzathief/scipy,aarchiba/scipy,scipy/scipy,WarrenWeckesser/scipy,lhilt/scipy,grlee77/scipy,lhilt/scipy,aarchiba/scipy,gfyoung/scipy,rgommers/scipy,zerothi/scipy,nmayorov/scipy,matthew-brett/scipy,scipy/scipy,person142/scipy,scipy/scipy,nmayorov/scipy,josephcslater/scipy,andyfaff/scipy,befelix/scipy,mdhaber/scipy,Stefan-Endres/scipy,Eric89GXL/scipy,perimosocordiae/scipy,rgommers/scipy,perimosocordiae/scipy,Stefan-Endres/scipy,e-q/scipy,jjhelmus/scipy,tylerjereddy/scipy,scipy/scipy,rgommers/scipy,gfyoung/scipy,vigna/scipy,aeklant/scipy,Eric89GXL/scipy,perimosocordiae/scipy,jjhelmus/scipy,endolith/scipy,vigna/scipy,befelix/scipy,zerothi/scipy,WarrenWeckesser/scipy,tylerjereddy/scipy,ilayn/scipy,lhilt/scipy,Eric89GXL/scipy,tylerjereddy/scipy,matthew-brett/scipy,person142/scipy,Stefan-Endres/scipy,grlee77/scipy,pizzathief/scipy,e-q/scipy,jor-/scipy,jamestwebber/scipy,person142/scipy,apbard/scipy,anntzer/scipy,befelix/scipy,josephcslater/scipy,apbard/scipy,jor-/scipy,pbrod/scipy,WarrenWeckesser/scipy,WarrenWeckesser/scipy,andyfaff/scipy,endolith/scipy,vigna/scipy,gfyoung/scipy,anntzer/scipy,grlee77/scipy,person142/scipy,matthew-brett/scipy,arokem/scipy,jakevdp/scipy,rgommers/scipy,aarchiba/scipy,endolith/scipy,jjhelmus/scipy,gertingold/scipy,tylerjereddy/scipy,pbrod/scipy,endolith/scipy,mdhaber/scipy,Stefan-Endres/scipy,anntzer/scipy,jakevdp/scipy,josephcslater/scipy,ilayn/scipy,jamestwebber/scipy,matthew-brett/scipy,jakevdp/scipy,zerothi/scipy,vigna/scipy,vigna/scipy,lhilt/scipy,tylerjereddy/scipy,nmayorov/scipy,mdhaber/scipy,pbrod/scipy,matthew-brett/scipy,arokem/scipy,lhilt/scipy,mdhaber/scipy,anntzer/scipy,mdhaber/scipy,jamestwebber/scipy,grlee77/scipy,josephcslater/scipy,aarchiba/scipy,Eric89GXL/scipy,endolith/scipy,pbrod/scipy,apbard/scipy,ilayn/scipy,apbard/scipy,perimosocordiae/scipy,jjhelmus/scipy,nmayorov/scipy,WarrenWeckesser/scipy,gertingold/scipy,andyfaff/scipy,Stefan-Endres/scipy,pizzathief/scipy,Stefan-Endres/scipy,befelix/scipy,aarchiba/scipy,rgommers/scipy,aeklant/scipy,jjhelmus/scipy,anntzer/scipy,e-q/scipy,anntzer/scipy,andyfaff/scipy,befelix/scipy,WarrenWeckesser/scipy