First implementation of Smoothed Seismicity tools

Implements a smoothed seismicity algorithm based on the Frankel (1995) approach. Contains:

i) hmtk.seismicity.smoothing.smoothed_seismicity.SmoothedSeismicity - a general wrapper class for implementing a smoothed seismicity analysis

ii) ...smoothing.utils - A set of utility functions including correction factors for time-varying completeness

iii) A kernels module currently implementing only (Frankel) isotropic Gaussian and a base class (others may follow)

iv) Tests for all of the above