Description

Add the GravimetricPowerDensity and VolumetricPowerDensity costs.

Issue reference

Fixes #529

Before you mark your PR as ready for review, please ensure that you've considered the following:

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.

[x] New Feature: A non-breaking change that adds new functionality.
[ ] Optimization: A code change that improves performance.
[x] Examples: A change to existing or additional examples.
[ ] Bug Fix: A non-breaking change that addresses an issue.
[ ] Documentation: Updates to documentation or new documentation for new features.
[ ] Refactoring: Non-functional changes that improve the codebase.
[ ] Style: Non-functional changes related to code style (formatting, naming, etc).
[ ] Testing: Additional tests to improve coverage or confirm functionality.
[ ] Other: (Insert description of change)

[ ] No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
[ ] All unit tests pass: $ nox -s tests
[ ] The documentation builds: $ nox -s doctest

You can run integration tests, unit tests, and doctests together at once, using $ nox -s quick.

[ ] Code is well-commented, especially in complex or unclear areas.
[ ] Added tests that prove my fix is effective or that my feature works.
[ ] Checked that coverage remains or improves, and added tests if necessary to maintain or increase coverage.

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