StaticTransform is a new interface that only include information for
transforming positions and directions. Transform is a subtype.

Added a getStaticTransform() method to Frame. Some preliminary testing
has shown a speed up of 1.5 compared to using getTransform() when
transforming positions. This is because StaticTransform.combine(...) can
ignore all the velocity and acceleration terms. (And that Orekit is very
good at caching frame transforms so the full precession model is rarely
evaluated.)

Also added some missing methods to make Transform.IDENTITY more
efficient.

Also added some tests.

For #903