Add function to identify connections in a snapshot

[marjanalbooyeh]

This PR adds a new function to cmeutils to identify angles and dihedrals from a snapshot's bond data.

The code mainly searches through the graph of bonds to find subgraphs that matches the angle and dihedrals sequence. The new hidden functions are almost copied from gmso's identify_connections with minor modifications.

ToDo:

• Add tests
• Move new hidden methods to a separate utils file (Do we need a utils directory for keeping the hidden methods in cmeutils? )

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6fa75c4) 96.01% compared to head (1b83345) 96.29%.
Report is 4 commits behind head on master.

❗ Current head 1b83345 differs from pull request most recent head 1929fd5. Consider uploading reports for the commit 1929fd5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   96.01%   96.29%   +0.28%     
==========================================
  Files           8        8              
  Lines         677      783     +106     
==========================================
+ Hits          650      754     +104     
- Misses         27       29       +2     

Files	Coverage Δ
cmeutils/gsd_utils.py	91.66% <98.11%> (+5.09%)	⬆️

[chrisjonesBSU]

LGTM! I tested it out with some PPS gsd files and it seems to work. I added a gsd file for PEKK and a unit test that checks for angles and dihedrals.