Begin implementing hit pointers in clusters
Add cluster container definitions to StEvent

FmsCluster container/iterators defined via macros in StContainers

Update .gitignore to not ignore them
Add clusters to StFmsCollection

Update .gitignore to not ignore source files
Use non-const hit pointers in TowerFPD

This is needed to eventually pass pointers to the original StFmsHits to
StFmsCluster
Fill cluster StFmsHit array

Fill hits from original StFmsHit array in StFmsCollection, as well as
the StFmsClHits created by the clustering code. We also to change
the StFmsCollection clusters member to a regular pointer vector, instead
of the STAR-defined StSPtrVecFmsCluster, to avoid clusters being deleted
twice, as the StFmsClusterCollection also does that.
Add StFmsClusters to StFmsCollection

Modify QA plots to use these clusters instead of StFmsClusterCollection.

Note that the use of StEvent::setFmsCollection() after adding clusters
was removed, as this was an unnecessary step, and actually led to
crashes (setting it deletes the original, but they shared hits!)
Delete StFmsClHit

Remove the class and StFmsClHitCollection. Stores pointers to the
original StFmsHits in StFmsCluster instead.
Replace StPtrVecFmsPoint typedef

Follow STAR convention and define it in StEvent/StContainers via
standard macros rather than using our own typedef.
Delete StFmsPointCollection

Use a simple vector of StFmsPoint pointers for the photon array in
StFmsCluster.

Note that it is not currently possible to use the STAR-standard
structures like StSPtrVecFmsPoint because of problems with including
StEvent/StContainers.h that I've not been able to resolve.

See https://www.star.bnl.gov/rt3/Ticket/Display.html?id=2816
Remove inheritance of HitCluster from StFmsCluster

This removes the compilation error due to the issue in
https://www.star.bnl.gov/rt3/Ticket/Display.html?id=2816
by avoiding inclusion (indirectly) of StContainers/StArray

There is a very slight change in numerical output for the photons,
which I think is just related to numerical precision - there arent'
any alterations in the algorithms used.
Use StArray containers in StFmsCluster

StPtrVecFitHit and StPtrVecFmsPoint containers for hits and photons
Use StArray container for StFmsCollection clusters

No longer add clusters to StFmsClusterCollection, as the StFmsCollection
now owns the clusters (in keeping with standard STAR practice).
Remove StFmsClusterCollection
Add StSPtrVecFmsPoint to StFmsCollection

Move ownership of points from StFmsCluster to StFmsCollection.
StSPtrVecFmsPoint takes care of deleting points, so it is no longer done
in the StFmsCluster destructor.

Modify QA maker to use StFmsCollection points instead of cluster points
Clean up StFmsCluster

Remove unused methods, add Doxygen comments and tidy formatting
Clean up StFmsCollection