Adds exporters for shakemap raster formats (geotiff/esri ascii) and contours (geojson)

Addresses #16 (closed)

A preliminary set of exporters is added to the Shakemap class to cover the initial needs of the EarthExplorer project:

  1. Transformation of the shakemap to raster and direct export to GeoTIFF or to ESRI ASCII file

  2. Retrieval of contours from the shakemap and export these to GeoJSON

For the handling of the raster format I am currently using rasterio (now added to the dependencies). Future merge requests may add features that require more direct use of the GDAL libraries for handling rasters (e.g. geopackage), in which case the GeoTIFF exporter may be replaced with one based on GDAL and the rasterio dependency removed.

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