Add new planetary ellipsoids and new naming scheme (#180)

Add ellipsoids for several solar system bodies. Names are now 
CamelCase and include the year at the end so we can have multiple
of the same body. 

Add the optional `comments` attribute to all classes to provide additional
 information for understanding the origin of the numbers used to 
instantiate the class (for example, many papers provide several different 
versions of the ellipsoid, and you need to choose one).

Add the `TriaxialEllipsoid.semimajor_axis_longitude` optional attribute. 
Previously, this parameter was found as an optional argument to the 
`geocentric radius` method but it's an intrinsic property of the ellipsoid 
and the coordinate frame, and will be necessary for all routines that make 
use of longitude coordinates. 

The Vesta ellipsoid was replaced since the there is not enough information 
in the paper to determine the GM, it is not stated if the ellipsoid is 
geocentric or offset from the center of mass, and the directions of the 
principal axes are not given. Instead, two new ellipsoids have been added: 
A biaxial `Vesta2017` and `VestaTriaxial2017`. The triaxial one is not aligned 
with the topography and gravity coordinate axes, and this is specified by 
the new `semimajor_axis_longitude` attribute.

---------

Co-authored-by: Leonardo Uieda <leo@uieda.com>

boule/__init__.py

@@ -6,7 +6,27 @@
 #
 # Import functions/classes to make the public API
 from ._ellipsoid import Ellipsoid
-from ._realizations import GRS80, MARS, MERCURY, MOON, VENUS, VESTA, WGS84
+from ._realizations import (
+    EGM96,
+    GRS80,
+    WGS84,
+    Callisto2024,
+    Ceres2018,
+    Charon2024,
+    Enceladus2024,
+    Europa2024,
+    Ganymede2024,
+    Io2024,
+    Mars2009,
+    Mercury2015,
+    Mercury2024,
+    Moon2015,
+    Pluto2024,
+    Titan2024,
+    Venus2015,
+    Vesta2017,
+    VestaTriaxial2017,
+)
 from ._sphere import Sphere
 from ._triaxialellipsoid import TriaxialEllipsoid
 from ._version import __version__

boule/_ellipsoid.py

@@ -24,7 +24,7 @@ class Ellipsoid:
 
     The ellipsoid is defined by four parameters: semimajor axis, flattening,
     geocentric gravitational constant, and angular velocity. It spins around
-    it's semiminor axis and has constant gravity potential at its surface. The
+    its semiminor axis and has constant gravity potential at its surface. The
     internal density structure of the ellipsoid is unspecified but must be such
     that the constant potential condition is satisfied.
 
@@ -59,6 +59,8 @@ class Ellipsoid:
         1984"`` (optional).
     reference : str or None
         Citation for the ellipsoid parameter values (optional).
+    comments : str or None
+        Additional comments regarding the ellipsoid (optional).
 
 
     .. caution::
@@ -133,6 +135,7 @@ class Ellipsoid:
     angular_velocity = attr.ib()
     long_name = attr.ib(default=None)
     reference = attr.ib(default=None)
+    comments = attr.ib(default=None)
 
     @flattening.validator
     def _check_flattening(self, flattening, value):