Add prep_interp_data to UserTiffData class.
Made edits to ensure that InterpGen works with the different user data classes. Added variable to interpGen() class to allow the stats table to be written to a file. Added a variable to interpGen() to allow the gridded data to be masked during stat calculations (this uses the xarray.mean() skipna variable). Added a grid to line notebook to the ClimateR and OPeNDAP example folders.