Refactor code to have a single Ellipsoid class (#616)

Refactor the ellipsoid code to have a single `Ellipsoid` class that can
represent any type of ellipsoid. Ditch the specific classes for each
ellipsoid type and the `create_ellipsoid` function. Update the forward
modelling code for gravity and magnetics. Make use of an extra rotation
matrix that aligns the x, y, z local coordinate system to the
ellipsoid's semiaxes in decreasing order, so `a >= b >= c`. Modify the
analytic solutions for oblate ellipsoids in such way that they are
defined by `a == b > c` (instead of `a < b == c` as Takahashi et al. and
Clark et al. do). Make rotation angles and `center` as optional
arguments for the `Ellipsoid` class. Fix numerical instabilities of
triaxial ellipsoids when they approximate prolate or oblate ellipsoids
(when two semiaxes lengths are close enough to each other). Update the
tests and add a few more for the new bits of code.

**Relevant issues/PRs:**

Part of #594

Author: santisoler

doc/api/index.rst

@@ -86,16 +86,12 @@ Magnetic fields:
     ellipsoid_magnetic
     prism_magnetic
 
-Ellipsoids:
+Geometric bodies:
 
 .. autosummary::
     :toctree: generated/
 
-    create_ellipsoid
-    OblateEllipsoid
-    ProlateEllipsoid
-    Sphere
-    TriaxialEllipsoid
+    Ellipsoid
 
 Layers and meshes:
 
@@ -174,4 +170,3 @@ Type hints
    :toctree: generated/
 
     typing.Coordinates
-    typing.Ellipsoid

harmonica/__init__.py

@@ -16,15 +16,7 @@
 from ._equivalent_sources.spherical import EquivalentSourcesSph
 from ._euler_deconvolution import EulerDeconvolution
 from ._forward.dipole import dipole_magnetic
-from ._forward.ellipsoids import (
-    OblateEllipsoid,
-    ProlateEllipsoid,
-    Sphere,
-    TriaxialEllipsoid,
-    create_ellipsoid,
-    ellipsoid_gravity,
-    ellipsoid_magnetic,
-)
+from ._forward.ellipsoids import Ellipsoid, ellipsoid_gravity, ellipsoid_magnetic
 from ._forward.point import point_gravity
 from ._forward.prism_gravity import prism_gravity
 from ._forward.prism_layer import DatasetAccessorPrismLayer, prism_layer

harmonica/_forward/ellipsoids/__init__.py

@@ -8,12 +8,6 @@
 Forward modelling of ellipsoids.
 """
 
-from .ellipsoids import (
-    OblateEllipsoid,
-    ProlateEllipsoid,
-    Sphere,
-    TriaxialEllipsoid,
-    create_ellipsoid,
-)
+from .ellipsoids import Ellipsoid
 from .gravity import ellipsoid_gravity
 from .magnetic import ellipsoid_magnetic