Add option to switch between hierarchy and averaging constraints in MP-based tracker alignment

The default behaviour is now to use averaging constraints after
problems with the fit convergence have been found when using
hierarchy constraints. This is explained in more detail in this
presentation by C. Kleinwort,

https://indico.cern.ch/event/337178/session/0/contribution/5/material/slides/0.pdf

Note that the implementation of the averaging constraints is still
approximate for the time being, but adequate for current purposes
and superior to the current version of the hierarchy constraints.
Merge pull request #6254 from mschrode/mp_averaging_constraints

Add option to switch between hierarchy and averaging constraints in MP-b...