WIP: Add TrackNProngVertex and TrackCompactVertex

This is a proposal for two new utilities:

• a new 'fast' N-prong vertex fit (called TrackNProngVertex)
• a class to store the result of the vertex fit such that p4 (and cov 7x7) can be computed after mass assignment to daughters). This would be the thing to store in the event store.

Both structures are templated on the floating point type and on the number of daughters: the latter can either be an integer with the actual number of daughters (in which case daughter information is stored in a std::array) or 'Dynamic' (standin for c++20 std::dynamic_extent, which didn't yet exists on my platform) in which case a vector is used.

There are still lot's of FIXME's in the code, but can be input for discussion in the next tracking meeting. (WH, 2019-04-01)