build_categoricalmassestimator should have z-score option and perform z-scoring by itself.

[michaeldeistler]

Currently, the build_categoricalmassestimator does not take arguments for z-scoring. Instead, since it is only actively used in MNLE, build_mnle takes care of z-scoring by passing a standarization net to the build_categoricalmassestimator here.

However, if build_categoricalmassestimator is supposed to be used standalone to estimate discrete densities, then this will not work. We should make the API of build_categoricalmassestimator consistent will also other build_xyz() functions and add z-scoring kwargs.