Add simple finegraining for UA systems

[jennyfothergill]

fixes #28

I've added some simple fine-graining for united atom systems that doesn't need a mapping. I've tested it on the first frame of one of my UA p3ht trajectories, but it might not work on a more distorted one.

• unit test
• test on later trajectory frame

[codecov]

Codecov Report

Merging #44 (a6ec58d) into main (f86bde9) will decrease coverage by 6.80%.
The diff coverage is 21.73%.

❗ Current head a6ec58d differs from pull request most recent head 02a8729. Consider uploading reports for the commit 02a8729 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
- Coverage   94.98%   88.19%   -6.80%     
==========================================
  Files           4        4              
  Lines         399      415      +16     
==========================================
- Hits          379      366      -13     
- Misses         20       49      +29     

Files	Coverage Δ
grits/utils.py	95.74% <75.00%> (-1.07%)	⬇️
grits/finegrain.py	62.13% <16.66%> (-26.58%)	⬇️

... and 1 file with indirect coverage changes