Major internal overhaul

This release changes a lot of internal code to improve future maintenance and allow for new features. The user facing API has not changed (meaning old code does not break).

New features:

• The Body class has 2 new methods called time_window_position and time_window_rotation to allow access to information about the times for which the position/rotation of the body is known.

Major Bugfixes:

• Unloading kernels works fine now.

First 'stable' version

This is the first version that works good enough to be released to the public.