Writer classes

I have,

• converted TextWriter and RootWriter from structs to classes,
• added RootInterferenceWriter to the project and also converted it from a struct to a class,
• created a new derived writer RootTripleAlphaWriter and renamed RootInterferenceWriter -> RootTripleAlphaInterferenceWriter
• moved them within the sim3a namespace,
• made them inherit from a template base class named Writer.
• modified the decay weight constructors so that the weight calculator can be specified at instantiation.
• updated the balamuth.cpp executable including its cml interface to run in three different modes: single partial wave, two partial waves with known mixing, two partial waves with unknown mixing

Still to do:

• Generalize implementation of writer classes to handle arbitrary numbers of final state particles, N, to be specified at instantiation (the current implementation is hardcoded to N=3)
• Make inclusion of Dalitz variables x,y in output root file optional
• Complete class docstring descriptions
• Implement unit tests for writer classes