Compression level for netCDF output

[YifanCheng]

If we run Metsim for a large area and at subdaily timestep, the output can be very large. It would be useful if we could choose the compression level of the output netCDF file.

[jhamman]

@YifanCheng - should be easy to add. Just mimic the 'out_precision' option and make sure it gets here:

MetSim/metsim/metsim.py

Lines 322 to 323 in b1dd17c

	encoding={'dtype': self.params['out_precision'],
	'_FillValue': cnst.FILL_VALUES['f8']})