Add an ellipsoid for Mars

[leouieda]

Based on the numbers is Ardalan et al. (2009). Include a plot of normal gravity on Mars in the tutorial as well.

Reminders

• [ ] Run make format and make check to make sure the code follows the style guide.
• [ ] Add tests for new features or tests that would have caught the bug that you're fixing.
• [ ] Add new public functions/methods/classes to doc/api/index.rst.
• [ ] Write detailed docstrings for all functions/methods.
• [ ] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[codecov[bot]]

Codecov Report

Merging #15 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #15   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines         101    102    +1     
  Branches        1      1           
=====================================
+ Hits          101    102    +1

Impacted Files	Coverage Δ
boule/realizations.py	100% <100%> (ø)	:arrow_up:

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