Updates based on 2nd JOSS review

[karllark]

The issues raised in the 2nd JOSS review (#222) are addressed here.

Developer documentation is added describing the base classes and the necessary components of each type of model. Writing this documentation motivated an update to the base classes, specifically moving the GMBase from grain_models.py to baseclasses.py and renaming it BaseExtGrainModel. This is more consistent with the structure of the package.

Small updates to the paper text based on the review.

The URL for G24 was updated to the published paper.

Closes #222.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.89%. Comparing base (010ca10) to head (2e36501).
Report is 51 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #224   +/-   ##
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  Coverage   99.89%   99.89%           
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  Files           8        8           
  Lines         919      919           
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  Hits          918      918           
  Misses          1        1           

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