Implements Ground Motion Intensity Conversion Equation module

A comprehensive shakemap suite may require the inclusion of shakemaps in terms of macroseismic intensity, as expressed here: #21 (closed)

This MR implements a sub-module for ground motion intensity conversion equations (GMICEs), including two commonly used models (Atkinson & Kaka, 2007; Worden et al., 2012). These models are the most widely used and generally applicable throughout the world. Uncertainty in the ground motion is incorporated into the uncertainty of macroseismic intensity using standard error propagation. The application of the GMICE is incorporated into the shakemap_from_quakeml workflow for Earthquake Explorer, meaning that it is now a standard output of the tools for this purpose.

Although the exact relation between GMICE may be region-dependent, there are still few models suitable for a shakmap application such as this. Nevertheless, a simple regionalisation is currently implemented, which adopts Atkinson & Kaka (2007) for the Global Stable tectonic region type, and Worden et al. (2012) everywhere else. This may be refined in future as new models are added.

The GMICE implementations are verified against test tables of data constructed using the original USGS shakemap software (https://github.com/usgs/shakemap/tree/master/shakelib/gmice).

@rizac Please take a look when you can

@nils @eggi FYI: this feature now means that you will find macroseismic intensity (MMI) in the suite of outputs of the shakemaps (even if it is not specified in the input IMTs). However, in order to apply this conversion the selected IMTs for the shakemaps must include one or more of PGA, SA(0.3), SA(1.0), SA(3.0) or PGV. If none of these are provided in the IMTs then an error will be raised.