Adds "set_mfd" function to modify Evenly Discretized MFD

The PR adds the first modification method to the EvenlyDiscretizedMFD class in order to allow the mfd to be
updated with a different set of min_mag, bin_width and occurrence_rates values. This change anticipates a change in the logic tree calculators to allow user to specfiy epistmic uncertainty on a source by providing directly a different set of incremental MFDs