Build-in import of USGS shakemaps

USGS provides observed ground-shaking map for many earthquakes that is nice to be used in the loss-calculator. These shakemaps are provided as hdf5 files.
The two main steps to have an importer in the loss-calculator for these shakemaps include:

1. Extracting the PGA, SA(0.3) & SA(1.0) from the HDF5 files; Rearrange them into a single dimension array that matches the grid; Also re-arrange the grid into a 1-d array.
2. Create the SA(0.6) using log-log interpolation as some fragility/vulnerability functions the SA(0.6) provided by ESRM20 is based on SA(0.6)

\fyi @ds @laurens