Expose metabolic constants in Animal model

[davidorme]

This PR:

• Moves the hard coded values for $E_s, \sigma, E_a$ in models.animals.scaling_functions.metabolic_rate to the new AnimalConstants configuration class.
• Adds the Bolzmann constant to the core constants.
• Passes all of these through to the metabolic_rate function via AnimalModel and AnimalCohorts constructors.
• Turns off parallel building in the docs CI step - crashes way too often to be useful.

Fixes #1134

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist

• Make sure you've run the pre-commit checks: $ pre-commit run -a
• All tests pass: $ poetry run pytest

Further checks

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works
• Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.35%. Comparing base (3d8ae81) to head (36bd818).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1135   +/-   ##
========================================
  Coverage    94.34%   94.35%           
========================================
  Files           70       70           
  Lines         7044     7051    +7     
========================================
+ Hits          6646     6653    +7     
  Misses         398      398           

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[TaranRallings]

This seems quite straightforward and a good idea for the future of consts. LGTM!