Set install path to build products dir
Constify assertion function
Add quaternion as product of rotation model (note: z is up through the pole)
Adding additional physical parameters.

Note that parameters such as radius do not really effect orbits per see,
except potential collisions. Collisions has to be sorted out external to
this library however, on the other hand, we may as well collect the
interesting physical parameters in one place.
Add functions to retrieve a list of all orbital objects, and to dynamically add
additional orbits.
Delete local paths in xcode file
Change semi-major axises from km to metres
Fix coordinate transformations for ELP2000 and Goffin.

The obl constant was in degrees. We now emphasise this properly. In addition,
the ELP2000 coordinates where in the ecliptic of the date, so they need to
be transformed into J2000 in order to resemble ICRF.
Kepler should transform coordinates based on parent object.