Add PowerDensity costs

[NicolaCourtier]

Description

Add the GravimetricPowerDensity and VolumetricPowerDensity costs.

Issue reference

Fixes #529

Review

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• Updated the CHANGELOG.md in reverse chronological order (newest at the top) with a concise description of the changes, including the PR number.
• Noted any breaking changes, including details on how it might impact existing functionality.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.05%. Comparing base (88071f1) to head (ec43450).
Report is 78 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #530   +/-   ##
========================================
  Coverage    99.05%   99.05%           
========================================
  Files           52       52           
  Lines         3582     3604   +22     
========================================
+ Hits          3548     3570   +22     
  Misses          34       34           

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[martinjrobins]

this looks great, thanks @NicolaCourtier. Only comment I would have is that it might be nice to put the mathematical expression for each cost in the docstrings, so it is clear what you are calculating when looking through the API docs.