Handle inter-dataset coordinate mismatch

[NoaMillsUSDA-ARS]

At least one dataset (PRISM) has data layers with coordinate values that don't perfectly align. If a user requests multiple variables from this dataset and does not specify a target crs or resolution (and thus GCDL does not call reproject_match()), then data_request_handler.py cannot correctly handle the merging of two xarray.DataArray objects into one xarray.Dataset object. This results in newly added variable layers containing entirely null values.