ENH: Untrack .c files generated by Cython

[ajgpitch]

It is not necessary to store these files in Github. They should be generated locally after a clone.
Tracking them leads to unnecessary commits, such as the last one in #719.

Can also mask issues in underlying .pyx files. I assume this is how the issues fixed in #697 must have made it into the devel branch. Otherwise tests could not have passed.

.c files added to .gitignore
cythonised c files untracked

Any future C files which are not generated by Cython can be added using

git add -f

The two C files that fit that description are not untracked by this PR.

[ekalosak]

This is great, thanks for improving the code sanitization 🙏
Problem is it's not passing CI (the build expects these .c files).
Once that's fixed, this LGTM, good engineering.

[ekalosak]

The only thing blocking this (otherwise correct) PR is to update the CI so it builds - and therefore doesn't require - the .c files you've removed. It's the right move, just incomplete.