Parser speed optimisation

[MauriceHendrix]

Description

Improved parser speed by delaying adding mathematics until connections are known.

I also added tests for them, and in a few places in the parser changed the error thrown to and assert as I didn't I couldn't come up with a way to make it every throw that error, let me know if I've missed something there.

Motivation and Context

Parsing wasn't as fast as it could be

Types of changes

Speed improvement

Documentation checklist

• [X] I have updated all documentation in the code where necessary.
• [X] I have checked spelling in all (new) comments and documentation.
• [X] I have added a note to RELEASE.md if relevant (new feature, breaking change, or notable bug fix).

Testing

• [x] Testing is done automatically and codecov shows test coverage
• [ ] This cannot be tested automatically

[codecov[bot]]

Codecov Report

Merging #303 into develop will increase coverage by 0.59%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #303      +/-   ##
===========================================
+ Coverage    98.60%   99.19%   +0.59%     
===========================================
  Files            6        8       +2     
  Lines         1361     1368       +7     
  Branches       299      301       +2     
===========================================
+ Hits          1342     1357      +15     
+ Misses           7        4       -3     
+ Partials        12        7       -5     

Impacted Files	Coverage Δ
cellmlmanip/__init__.py	100.00% <100.00%> (ø)
cellmlmanip/_config.py	100.00% <100.00%> (ø)
cellmlmanip/model.py	100.00% <100.00%> (+0.21%)	:arrow_up:
cellmlmanip/parser.py	99.44% <100.00%> (+2.05%)	:arrow_up:
cellmlmanip/units.py	98.41% <100.00%> (ø)

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