Optimize the coordinate conversions and covariance matrix computations of the EigenStepper (was ACTS-340)

Original author Hadrien Benjamin Grasland @hgraslan

The current Propagator design relies a lot on frequent track parameter coordinate conversions and covariance matrix computations, which are not particularly optimized in the EigenStepper.

The importance of these operations is likely to go down in the future, but they plays a key role in today's performance benchmarks, and the easiest optimizations come with unchanged or increased code clarity. So I think we should go for these anyway.