Make methods and arguments const where possible

As discussed in #19
Use STL list instead of TObjArray for tower lists

Affects both StFmsTowerCluster and StFmsClusterFitter. This simplifies
iterating over the lists (via BOOST_FOREACH) and makes certain
operations (e.g. sorting) easier.

Also apply const-correctness more thoroughly to some private functions
in StFmsClusterFitter.

Correct function name and comments around sortTowersEnergyAscending(),
which actually sorts in descending order
Remove unused helper function

Also replace a couple of functors with functions to simplify
implementation
Remove StFmsTower Compare method

Not really needed now that ROOT containers aren't sorted, as it is more
easily implemeted as a helper function to use with STL list
Replace some helper functors with functions
Modify StFmsCluster method/member naming

Now follows STAR conventions as in #18
Modify StFmsPoint method/member naming

Now follows STAR conventions as in #18
Make StFmsHit in StFmsTower const

We don't (and shouldn't) modify the original StFmsHits from StEvent.
See #19
Use const pointers throughout StFmsGeometry

We shouldn't modify the values we read from the database. See #19
Use const geometry in clustering code

Closes #19