WIP: First draft of plate velocity computations

This draft does not, yet, compile and needs some further pimping to be
HyTeG-style. It is based on:

https://gitlab.lrz.de/marcus.mohr/plates-refactoring/-/tags/HYTEG_INCLUSION_CANDIDATE_1
31 jobs for mohr/plateVelocities in 23 seconds (queued for 2 seconds)