MAINT: Fix sign-compare warnings in umath_linalg.
MAINT: Fix sign-compare warnings in umath_linalg.

The sign-compare warning is enabled for python 3.5+ builds and
the warnings have been causing wheel build failures on travis ci.

The replacements here of `size_t` and `ptrdiff_t` by `npy_intp` should
be safe given that the sizes of numpy arrays in bytes are specified by
those types. A bigger problem is that most BLAS libraries are compiled
with smaller integer types. I don't know where that is checked, or even
if it is checked.

grlee77/numpy,MSeifert04/numpy,charris/numpy,MSeifert04/numpy,shoyer/numpy,mattip/numpy,charris/numpy,seberg/numpy,ahaldane/numpy,WarrenWeckesser/numpy,pdebuyl/numpy,charris/numpy,MSeifert04/numpy,endolith/numpy,tynn/numpy,grlee77/numpy,pizzathief/numpy,mhvk/numpy,Eric89GXL/numpy,rgommers/numpy,Eric89GXL/numpy,anntzer/numpy,pbrod/numpy,ahaldane/numpy,pbrod/numpy,WarrenWeckesser/numpy,mattip/numpy,mattip/numpy,shoyer/numpy,mhvk/numpy,simongibbons/numpy,Eric89GXL/numpy,jorisvandenbossche/numpy,pdebuyl/numpy,abalkin/numpy,numpy/numpy,shoyer/numpy,grlee77/numpy,madphysicist/numpy,MSeifert04/numpy,seberg/numpy,numpy/numpy,jakirkham/numpy,simongibbons/numpy,seberg/numpy,gfyoung/numpy,jorisvandenbossche/numpy,madphysicist/numpy,endolith/numpy,jakirkham/numpy,rgommers/numpy,pizzathief/numpy,shoyer/numpy,gfyoung/numpy,mhvk/numpy,rgommers/numpy,pbrod/numpy,ahaldane/numpy,madphysicist/numpy,rgommers/numpy,WarrenWeckesser/numpy,charris/numpy,grlee77/numpy,mhvk/numpy,jorisvandenbossche/numpy,jakirkham/numpy,tynn/numpy,pizzathief/numpy,jakirkham/numpy,simongibbons/numpy,ahaldane/numpy,Eric89GXL/numpy,mhvk/numpy,jakirkham/numpy,numpy/numpy,pizzathief/numpy,gfyoung/numpy,WarrenWeckesser/numpy,anntzer/numpy,madphysicist/numpy,endolith/numpy,pbrod/numpy,simongibbons/numpy,mattip/numpy,ahaldane/numpy,endolith/numpy,pdebuyl/numpy,simongibbons/numpy,shoyer/numpy,pizzathief/numpy,anntzer/numpy,abalkin/numpy,anntzer/numpy,jorisvandenbossche/numpy,pdebuyl/numpy,abalkin/numpy,WarrenWeckesser/numpy,numpy/numpy,madphysicist/numpy,seberg/numpy,MSeifert04/numpy,tynn/numpy,jorisvandenbossche/numpy,grlee77/numpy,pbrod/numpy