Catalogue histograms1

Adds methods to the hmtk.seismicity.catalogue class and the hmtk.seismicity.utils module to investigate catalogue by means of 1-D and 2-D histograms. Histograms also include bootstrapping to allow for uncertainties in data to be
incorporated. 

Three methods added to hmtk.seismicity.catalogue.Catalogue class are:
1) Depth histograms
2) Magnitude - Depth histograms
3) Magnitude - Time histograms