Add the volume property for the ellipsoid (#132)

Checked against approximate values reported at
https://en.wikipedia.org/wiki/Earth

boule/_ellipsoid.py

@@ -101,6 +101,8 @@ class Ellipsoid:
     8.2094437949696e-02
     >>> print(f"{ellipsoid.mean_radius:.4f} m")
     6371008.7714 m
+    >>> print(f"{ellipsoid.volume * 1e-9:.5e} km³")
+    1.08321e+12 km³
     >>> print(f"{ellipsoid.gravity_equator:.10f} m/s²")
     9.7803253359 m/s²
     >>> print(f"{ellipsoid.gravity_pole:.10f} m/s²")
@@ -217,6 +219,15 @@ def mean_radius(self):
         """
         return 1 / 3 * (2 * self.semimajor_axis + self.semiminor_axis)
 
+    @property
+    def volume(self):
+        r"""
+        The volume bounded by the ellipsoid.
+        Definition: :math:`V = \dfrac{4}{3} \pi a^2 c`.
+        Units: :math:`m^3`.
+        """
+        return (4 / 3 * np.pi) * self.semimajor_axis**2 * self.semiminor_axis
+
     @property
     def _emm(self):
         "Auxiliary quantity used to calculate gravity at the pole and equator"