Made Model.get_value() evaluate states

[MichaelClerx]

Description

See #278

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist:

• [x] I have updated all documentation necessary.
• [x] I have checked spelling in (new) comments.

Testing

• [X] Testing is done automatically and codecov shows test coverage

[codecov[bot]]

Codecov Report

Merging #279 into develop will increase coverage by 0.04%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #279      +/-   ##
===========================================
+ Coverage    97.52%   97.57%   +0.04%     
===========================================
  Files            7        7              
  Lines         1336     1359      +23     
  Branches       287      295       +8     
===========================================
+ Hits          1303     1326      +23     
  Misses          15       15              
  Partials        18       18              

Impacted Files	Coverage Δ
cellmlmanip/model.py	99.78% <100.00%> (+0.01%)	:arrow_up:

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[MichaelClerx]

Sorry did another slight update @jonc125 :D