Fix gradient calculation from model.simulateS1

[BradyPlanden]

Description

1. Updates the dy calculation within model.simulateS1
2. Refactors and simplifies cost._evaluateS1 for fitting costs
3. Updates corresponding tests

Issue reference

Fixes #259

Review

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.33%. Comparing base (80514d0) to head (47767c7).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #260      +/-   ##
===========================================
+ Coverage    95.13%   95.33%   +0.19%     
===========================================
  Files           36       36              
  Lines         1994     1971      -23     
===========================================
- Hits          1897     1879      -18     
+ Misses          97       92       -5     

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[NicolaCourtier]

Thanks for fixing the bug and tidying up the calculations- great work @BradyPlanden! Let's merge and release!