Fixes to the occurrence calculators (a-values)

Fixes some issues raised in the occurrence calculators. Namely the following:

1. In b_maximum_likelihood, a-value is no longer calculated as the weighted mean, but is determined using the estimator of Weichert (1980) and McGuire (2004)

2. Modifies the requirements in the occurrence config

3. Updates tests

4. Adds cumulative rate completeness analysis into the plotting tools. This is an interactive gui-based method in which the user defines the completeness tables by looking at cumulative rates of events within each specified magnitude bin