Remove the SpiceKernel class

[dahlend]

The SpiceKernel class is a hold over from having the spice interpreter written in python. It existed to provide a singleton in the python framework, allowing the loaded spice information to be loaded only a single time.

This has not been required since the code was moved to rust, and now it remains as an class which is used as a static namespace. This is non-python and is odd for new users.

The goal is to remove this entirely and reduce all of its methods to be functions in the spice submodule.

   import neospy

   # neospy.SpiceKernels.state("Earth", neospy.Time.j2000())
   # becomes:
   neospy.spice.state("Earth", neospy.Time.j2000())