Interpolation should precede Calov & Greve formula

In the PDD calculation, interpolation should precede the computation of effective temperature for melt, otherwise we compute slightly too high PDD sums.
Two very distinct changes

* The sigma parameter in the Calov and Greve integral is now passable as a numpy array, and automatically interpreted by the netcdf operator.

* The name of dimensions in the input file can be other than 'x' and 'y'. They are read from the temperature variable.
Add an option to force constant temperature stdev

This is automatically activated when using --pdd-std-dev on the command-line.
Add coordinate variables to output files

Variables whose names match dimension names are copied to output file.
Modify handling of standard deviation

Use constant value if provided, otherwise look for air_temp_stdev in the input file, and otherwise use scalar zero as default.