WIP: Added multi-indexed hit container.

(This might be the most incomplete MR from my side, and I'm sorry for this. However, people are dependent on this work and I do not see myself having the time to change the implementation further these days. The API should be final enough.)

More general implementation to the HitContainers. Up so far, we have had different HitContainers for different levels of nesting: the current Outer Tracker has a HitContainer<3,4,9>, while for the FT there is a HitContainer1D<12>. This MultiIndexHitContainer supports any level of nesting.

I have tried to include @raaij 's latest changes to the sorting. With this I ran tests from @rquaglia and @chasse with Brunel succesfully.

(Note to self: When you change branches in a MR, it removes all of your previously written description... )