Add orbital elements processing

[attipaci]

We should be able to compute the positions of Solar-system bodies from orbital elements.

The proposed approach can be summarized as:

• Add a new entry to enum novas_object_type, e.g. NOVAS_ORBITAL_OBJECT, specifying that the object in question is defined by orbital elements.
• Create new novas_orbital structure to store orbital elements.
• Add a orbit field to object. (This will make object a little bulkier, but it does not break a thing...)
• Add function to convert novas_orbital to x,y,z coordinates and velocity vectors.
• ephemeris can then use above function for NOVAS_ORBITAL_OBJECT type objects.

Once we have such a framework, we can parse orbital data e.g. from MPCORB data format(s)...