New body.frame and body.shape submodules

[altairgomes]

In this Pull Request, I add two new features: the "frame" and "shape" submodules:

• In frame:

  • We define a new class for the orientation of the body (PlanetocentricFrame), giving its rotation velocity, pole coordinates, and precession parameters as defined by Archinal et al. (2018). With it, we can transform to other reference frames using Astropy.
  • The definitions of the orientation parameters in Archinal et al. (2018) are hardcoded. The user can overwrite it by providing its own frame.
  • With it, we process and rotate the body to find its orientation at any epoch (must consider light time).

• In shape:

  • We define the new Shape3D class where we can provide to the body a wavefront file with its 3D shape. An image corresponding to the texture can also be provided.
  • We can plot the 3D shape on the tangent plane considering the sub-observer coordinates, its pole orientation, and the solar illumination.
  • We can compute the limb of the rotated 3D shape to use it in the fitting process. This limb now requires a dependency on shapely.

The fitting procedure with the limb feature is not yet implemented in Occultation but is made available in occultation.fitting. At the moment, it can only fit the center coordinates.