Add basic template for StMuFmsCluster class

Update .gitignore to ignore remaining files in StMuDSTMaker
Add basic StMuFmsCluster source file
Add accessor methods to StMuFmsCluster

Use the same return type and naming as StMuFmsHit where methods are
analagous
Change StMuFmsCluster integers to unsigned short

Matches convention with StMuFmsHit
Add setter methods for simple fields