Add GRB support

[apizzuto]

Add a separate class for doing GRB analyses. Liz Friedman has a list of subtle differences that will need to be accounted for, including:

• different dataset (include zenith_smoothing)
• Effective trials factor calculation
• different precomputed background trials

[apizzuto]

Should mention that this is in progress in https://github.com/icecube/FastResponseAnalysis/blob/enhancement/restructure/fast_response/FastResponseAnalysis.py, which is a dev branch that currently does not work at all