Adds logging of shakemap statistics in shakemap_from_quakeml workflow
Addresses #20 (closed)
Adds statistics to the ShakemapWorkflowResult class to log some summary statistics about the shakemaps themselves. This could make it easier in the future to, for example, filter or sort some shakemap results by maximum ground motion or average ground motion. Initially only the maximum, minimum, geometric mean and log IQR of the ground motion fields are stored. Other information could be added in future.