Remove CPP and use only C in get neighbors optmization

[chc273]

Summary

Recently, we have seen some platform-dependent errors caused by this cython extension.

The known error is "illegal instructions (core dumped)" which cause crash of python. This error was found on conda installations. In particular, I have encountered issues on ubuntu 18.04.4 LTS (Matt raised this issue to me), and the github action VM macOS-latest, python3.7. Such errors however do not show up if pymatgen is installed using pip or from source. Interestingly, it also does not show up on github action VM macOS-latest, python3.8.

This PR is an attempt to fix the issue. My suspicion is that this is related to C++ compilation since it requires the platform-dependent flags. In this PR, I have removed all CPP-dependent codes and replace them with C only codes.

[mkhorton]

Many thanks for addressing this @chc273.

For documentary purposes, I'm just noting we also noticed this issue on conda installs via Binder (which runs Ubuntu), and previously we also had some issues with Linux wheels via PyPI on #1868 that I think might be related.

[chc273]

Many thanks for addressing this @chc273.

For documentary purposes, I'm just noting we also noticed this issue on conda installs via Binder (which runs Ubuntu), and previously we also had some issues with Linux wheels via PyPI on #1868 that I think might be related.

Ah, I missed that issue. Thanks for bringing that to my attention... I hope this fix will do the job, otherwise, it could take much more time, since it is not very obvious where the problem is and the problem seems to be very dependent on the installation methods..

[mkhorton]

I'm pretty confident that a switch to pure-C will fix this.

In that other issue, we were trying a new method of building PyPI wheels, so I'd assumed there was an issue with the build process rather than an issue in the code itself. However, since we're seeing it on conda too, it's clear there must just be a compatibility issue across Linux distributions with the current code.