Remove side-effects from convolution.convolve()
Remove side-effects from convolution.convolve()

This extends #7303.
See [this comment](https://github.com/astropy/astropy/pull/7293#issuecomment-391184153) in #7293

Signed-off-by: James Noss <897e4a798cfa3e609063a099c3c4ac849e1a5839@stsci.edu>

aleksandr-bakanov/astropy,larrybradley/astropy,MSeifert04/astropy,pllim/astropy,saimn/astropy,StuartLittlefair/astropy,pllim/astropy,lpsinger/astropy,larrybradley/astropy,stargaser/astropy,lpsinger/astropy,StuartLittlefair/astropy,pllim/astropy,astropy/astropy,stargaser/astropy,larrybradley/astropy,astropy/astropy,bsipocz/astropy,MSeifert04/astropy,mhvk/astropy,astropy/astropy,mhvk/astropy,saimn/astropy,saimn/astropy,lpsinger/astropy,aleksandr-bakanov/astropy,aleksandr-bakanov/astropy,dhomeier/astropy,bsipocz/astropy,astropy/astropy,saimn/astropy,dhomeier/astropy,mhvk/astropy,StuartLittlefair/astropy,lpsinger/astropy,dhomeier/astropy,MSeifert04/astropy,mhvk/astropy,bsipocz/astropy,pllim/astropy,larrybradley/astropy,mhvk/astropy,astropy/astropy,pllim/astropy,dhomeier/astropy,dhomeier/astropy,MSeifert04/astropy,aleksandr-bakanov/astropy,lpsinger/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,larrybradley/astropy,saimn/astropy,bsipocz/astropy,stargaser/astropy,stargaser/astropy