DOC: Add a doctest to calculate density profile

[justincolinsmart]

Pull request type

• ReadMe, Docs and GitHub updates

Checklist

• Docs have been reviewed and added / updated
• 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

• No

[Gui-FernandesBR]

Thank you for your contribution, @justincolinsmart ! Good to see you here.

Overall, the PR looks good, straight to the point,

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

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

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