CHT: Set native and target magnitude and handle missing measure uncertainty info

[matley]

The user should be able to specify a target and a native magnitude scale.

E.g.

emr = EmpiricalMagnitudeRelation(native='mb', target='Mw')

It should be possible to specify if the native magnitude string is case-sensitive or not (as in some cases mb != Mb).

emr = EmpiricalMagnitudeRelation(native='mb', target='Mw', case_sensitive=True)

It should be possible to use simple wildcards or regular expression, e.g. (to consider mb, mb1x, etc.):

emr = EmpiricalMagnitudeRelation(native='mb*', target='Mw')

The user should be able to specify how to handle missing uncertainty.

1) The user should be able to specify when an event should be discarded from the results when the uncertainty is missing.
E.g.

emr.set_missing_uncertainty_handling( eqcatalogue.DISCARD )

2) The user should be able to specify a default value for missing info
emr.set_missing_uncertainty_handling( eqcatalogue.DEFAULT(4) )

3) When uncertainty infos are missing the user can specify to use the average/maximum/minimum uncertainty present in the measures for the same event.
emr.set_missing_uncertainty_handling( eqcatalogue.EVENT_MAXIMUM )

Provide tests for each use case.