Accept ArcGIS ASCII grid input to generate_param_file

[scottza]

Add support for ArcGIS ASCII grids in the pytopkapi.parameter_utils.create_file.generate_param_file function. Currently only GTIFF is accepted.

See mailing list discussion https://groups.google.com/forum/?hl=en&fromgroups=#!topic/pytopkapi/J7HGNcNR37E