Weight classes

This merge request completes review and cleaning of

• DecayWeight
• BalamuthWeight
• BalamuthInterferenceWeight (renamed to BalamuthMultiWaveWeight)

For each class, the following has been done,

• license text
• doc strings for class and methods
• unit tests

Additionally, the following improvements/changes have been made:

1. Added protected virtual method CalculateWeight in DecayWeight, which is overwritten in the child classes. With this approach the application of the overall scaling factor can be handled entirely by DecayWeight.

2. Renamed BalamuthInterferenceWeight to BalamuthMultiWaveWeight (to avoid confusion with intereference due to boson symmetrisation) and fixed a major bug in the coherent summation of amplitudes.

3. Generalized BalamuthMultiWaveWeight to be able to handle superposition of arbitrary number of partial waves and not just two. (I have retained the old constructor interface for instantiating with only two partial waves.)

4. Added a second constructor in BalamuthWeight to simplify the instantiation for 12C->3a decays.

5. Added capacity to Nucleus class for keeping track of which excitation level the nucleus is currently in.