Gplately save shp (#205)

* added saving to shapefile as an option for gpts

added .shp as a save option for gpts, so that data can go straight into qgis rather than through gplates first

* added shp to error message as well

gplately/reconstruction.py

@@ -1884,12 +1884,12 @@ def save(self, filename):
             df = self._get_dataframe()
             df.to_xml(filename, index=False)
 
-        elif filename.endswith(".gpml") or filename.endswith(".gpmlz"):
+        elif filename.endswith(".gpml") or filename.endswith(".gpmlz") or filename.endswith(".shp"):
             self.FeatureCollection.write(filename)
 
         else:
             raise ValueError(
-                "Cannot save to specified file type. Use csv, gpml, or xls file extension."
+                "Cannot save to specified file type. Use csv, gpml, shp or xls file extension."
             )
 
     def rotate_reference_frames(