write soy.masses.Mass

[ArcRiley]

This is the base class for other mass classes (Box Sphere etc)

Wrap ODE's mass functions and the dMass storage in such a manner that other classes may inherit and overwrite (via cdef functions/etc) the standard behavior, while exposing to Python the .mass .center and potentially .inertia

Body class should be able to utilize these internals directly without having to access ODE calls and the defaults should provide the most basic functionality akin to what Body currently provides internally.