Move constraints code to new submodule

[tsmbland]

Just figured optimisation.rs is a bit big and worth breaking it up a bit. Especially as we'll probably want to add more constraints in the future

[codecov]

Codecov Report

Attention: Patch coverage is 95.38462% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.10%. Comparing base (fc61563) to head (1b6ac70).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/simulation/optimisation/constraints.rs	95.38%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #470   +/-   ##
=======================================
  Coverage   95.10%   95.10%           
=======================================
  Files          34       35    +1     
  Lines        4947     4947           
  Branches     4947     4947           
=======================================
  Hits         4705     4705           
  Misses        120      120           
  Partials      122      122           

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[alexdewar]

Seems sensible 👍