Document Hype subbasin should match

The Hype model uses subbasins. The forcing and parameter set should use the same subbasins.

The documentation for the `shape_file` argument of `ewatercycle.forcing.generate() ` should mention that the shape file should have the same subbasins as the subbasins used in the parameter set files like [GeoData.txt](http://www.smhi.net/hype/wiki/doku.php?id=start:hype_file_reference:geodata.txt).

Also to use `Hype.get_value_at_coords()`, the GeoData.txt needs to define latitude and longitude columns in GeoData.txt.

Start with the documentation for ewatercycle.forcing.generate() and its shape_file argument, then review the GeoData.txt reference linked in the issue. Document that the shape file and parameter-set files use the same subbasins, and note that GeoData.txt needs latitude and longitude columns for Hype.get_value_at_coords().

Written by the indexing model from the issue text.