Remove choice of integration method.

I don't see the use of using anything but 'rectangle', which comes down to a basic sum over the array.
Separate refreezing of snow and ice
Change (again) handling of standard deviation.

* PDDModel.__call__ now requires stdv argument.
* PDDModel.nco() tries to read stdv from file, and use zero if not found.
* Command-line option --pdd-std-dev is gone.
* r.in.pdd looks for option stdv and uses zero if not provided.
Allow arbitrary number of records in GRASS script.

Also expand shapes (1, x, y) to (t, x, y) in the main program.