Report empty detector map as an error, not just a warning

(cherry picked from commit d92b74024d4c41c98795e2d89d5d3f3b52ea7724)
Bugfix: Don't report error if THaHelicityDet can't find its database file

All ReadDatabase functions should behave in this way if they only read
optional paramaters, as this one does.

(cherry picked from commit 6482c5abfa670aee159cef4e1c699f4c632f8f4e)
Let Git also ignore dictionary files ending in .cxx, as in the SDK
Add cluster size to MCTrackPoint

This is intended to be the size of the reconstructed cluster (group of
hits/strips/wires etc.) found closest to the MCTrackPoint. It could
also be some other counter, like hit multiplicity, as appropriate.
Move event weight from MCTrack to SimDecoder event data

There should only be one MC weight per event, unless I misunderstand
something.
Initialize fit results in MCTrack to kBig, not 0.
Support defining RTTI variables that are specific elements of fixed-sized arrays