DOC: Add a doctest to calculate density profile

[justincolinsmart]

Pull request type

• [X] ReadMe, Docs and GitHub updates

Checklist

• [X] Docs have been reviewed and added / updated
• [X] Lint (black rocketpy/ tests/) has passed locally

Current behavior

Previously environment.calculate_density_profile lacked doctest

New behavior

Now environment.calculate_density_profile now has two doctest's, one for sea level and another at 1000m

Breaking change

• [X] No

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 72.97%. Comparing base (1b9e950) to head (e5d6fce). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #591 +/- ## ======================================== Coverage 72.97% 72.97% ======================================== Files 57 57 Lines 9596 9596 ======================================== Hits 7003 7003 Misses 2593 2593 ```

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