Add system and utils unit tests

[marjanalbooyeh]

This PR adds more tests for the following modules:

1- All the setter methods for reference values in System base class. Cases with ReferenceUnitError are being tested too.

2- ScaleEpsilon and ScaleSigma in utils actions.

3- xml_to_gmso_ff in ff_utils.

[chrisjonesBSU]

@marjanAlbouye While we're making some small changes to the reference units attributes and setters, what do you think about adding a check for the correct dimensions?

Unyt units have a dimensions parameter that return (mass), (length) the dimensions for energy is (length)**2*(mass)/(time)**2

For example, we should throw an error if someone tries setting reference_length to a unit that doesn't have dimensions of (length)

[codecov]

Codecov Report

Merging #34 (006c838) into main (bb841e0) will increase coverage by 2.20%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   89.17%   91.38%   +2.20%     
==========================================
  Files          18       18              
  Lines        1303     1312       +9     
==========================================
+ Hits         1162     1199      +37     
+ Misses        141      113      -28     

Files Changed	Coverage Δ
hoomd_organics/base/system.py	88.75% <100.00%> (+6.83%)	⬆️

... and 4 files with indirect coverage changes

[marjanalbooyeh]

@chrisjonesBSU
I added some checks in system.py to make sure the reference units have correct dimensions. Unit tests have been added accordingly.

[marjanalbooyeh]

ToDo for me: Move all the checks related to units and their dimensions to utils to avoid having duplicated code. Also, allow unit strings with / character such as "kcal/mol".

[chrisjonesBSU]

ToDo for me: Move all the checks related to units and their dimensions to utils to avoid having duplicated code. Also, allow unit strings with / character such as "kcal/mol".

Do you want to do that in this PR, or merge this and make a separate PR that streamlines the reference unit setters?

[marjanalbooyeh]

ToDo for me: Move all the checks related to units and their dimensions to utils to avoid having duplicated code. Also, allow unit strings with / character such as "kcal/mol".

Do you want to do that in this PR, or merge this and make a separate PR that streamlines the reference unit setters?

I was planning to do it in this PR but this PR is getting big. I'll merge this one and start a new one.