Add CG_System class

[jennyfothergill]

Fixes: #16

OK, so I've been going round and round for how to fix this. Chris mentioned the CG_Compound class would still be useful to him for building up systems, so it makes sense to keep it because it works pretty well with a small system and mbuild has useful functions for initializing systems. In addition to this class, maybe we create a separate class for if a user just wants to use this package for msibi--a workflow something like:
(atomistic gsd) ->
(freud clustering - get the molecules) ->
(openbabel molecule - SMARTS matching) ->
(scale out that mapping to other molecules) ->
(coarse gsd)
In which case there's no reason to get mbuild involved. Maybe we want to keep this simple and not specific to gsd; any system of points, types, bonds, and box should work.

Maybe additionally we can get some speedup by not forcing any optimization steps when placing fine-grain structures... instead these can be optional.

TODO:

• a function to write coarse structures to gsd.
• a way to visualize that this new method is correct? Use OVITO 😅
• Add unit tests including testing a multicompound gsd as I'm not sure how that'll work
• Document the assumptions of the CG_System namely:
  • molecules (clusters determined using bonds) of the same length are assumed to have the same structure
  • structures which are the same are expected to be added to the system in a chunk and follow the same pattern (this is the way planckton builds up systems) for example if a system contains 200 of each water and hexane it is expect to be 200 water then 200 hexane e.g. O, H, H, O, H, H, ..., C, C C, ...
• Add particle images

[codecov]

Codecov Report

Merging #19 (5403c4b) into master (f3ae375) will increase coverage by 0.57%.
The diff coverage is 97.88%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
+ Coverage   96.33%   96.91%   +0.57%     
==========================================
  Files           4        4              
  Lines         218      356     +138     
==========================================
+ Hits          210      345     +135     
- Misses          8       11       +3     

Impacted Files	Coverage Δ
grits/utils.py	96.73% <93.33%> (-3.27%)	⬇️
grits/__init__.py	100.00% <100.00%> (ø)
grits/coarsegrain.py	97.96% <100.00%> (+1.81%)	⬆️

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