Add shakemaps for macroseismic intensity

Needed for i) EarthquakeExplorer, ii) use cases where the shakemap may be linked to building damage or vulnerability calculations based on macroseismic intensity.

There are two options for doing this:

1. Define a global regionalization for intensity prediction equations (IPEs)

2. Use a ground motion intensity conversion equation (GMICE)

Option 1 would be preferable if there were a sufficiently large number of intensity prediction equations to cover all tectonic region types in order to define these everywhere. In reality, there are considerably fewer IPEs in the literature than GMPEs, some are very localised (to a specific country or region) and not all cover all of the different tectonic environments needed. Therefore the alternative is to use the GMICEs. There are several available in the literature (e.g. Worden et al., 2012; Faenza & Michelini, 2010; Caprio et al., 2015) and though there are some regional differences in GMICEs, these are more appropriate for application worldwide.

GMICEs usually describe the relationship between PGA, PGV and/or SA and macroseismic intensity. Though PGV is often seen as the better predictor of intensity, not all GMPEs selected in the default regionalisation actually define PGV, therefore it would need to be inferred from PGA or SA. Given that this conversion would carry larger uncertainty than using PGA - intensity, I would stick with just the PGA to intensity conversions.