Add shakemap summary statistics into ShakemapsWorkflowResult

For the case where a user may be building an archive of shakemaps (e.g. EarthquakeExplorer) it can be useful to have some summary statistics about the shakemaps (e.g. mean acceleration, max acceleration etc.) included into the ShakemapsWorkflowResult object. This could make it easier to store attributes that could be used for filtering/searching the shakemaps subsequently, rather than having to retrieve the values from the rasters