Add mass, mean_density, and volume_equivalent_radius properties

[MarkWieczorek]

This PR addresses the issue https://github.com/fatiando/boule/issues/164 concerning the addition of mass, mean_density, and volume_equivalent_radius properties to the ellipsoid classes. The following changes were made:

• Created a top level file _constants.py to house to the gravitational constant G that is imported into the three ellipsoid classes.
• Added the properties mass, mean_density, and volume_equivalent_radius to each of the three ellipsoid classes.
• Clarified the docstrings of mean radius: This is, in fact, not the mean radius of the sphere (i.e., the degree 0 coefficient of the shape), but rather is the mean of the semi-axes (a + b + c)/3. This is defined as R_1 in WGS84 Moritz1988.

Comments

• The mathematical equation for the volume_equivalent_radius is (self.volume * 3 / 4 / np.pi)**(1 / 3). If we expanded the definition of self.volume we could probably save a couple multiplications (but at the expense of readability).
• I don't know if we need to say "Added for compatibility with pymap3d." in the Sphere docstrings. For me, these quantities are defined so that all classes take the same parameters.

And, I'm happy to modify anything to conform to your coding standards that are higher than mine!

Relevant issues/PRs: Fixes #164

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[leouieda]

I agree that we don't need to worry about multiplying a few more times. This is not something called in a loop.

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