Dataset interpolation efficiency

[johnomotani]

At the moment interpolate_parallel is implemented for BoutDataArray, and BoutDataset.interpolate_parallel() iterates over the member variables. This means that the coordinates get interpolated many times. It would probably be more efficient to implement either as a utility function that can be called on either a DataArray or a Dataset and then is called in BoutDataArray.interpolate_parallel() and BoutDataset.interpolate_parallel() on self, or to implement for BoutDataset (which should be mostly a copy-paste of the BoutDataArray.interpolate_parallel() code, and then in BoutDataArray.interpolate_parallel() convert the DataArray to a Dataset with one member variable, call the BoutDataset method, and convert back to a DataArray.