Pypdd

This is the first Python 3 release of PyPDD, also including:

* easy installation with ``pip install pypdd``,
* bug fixes in the GRASS GIS module.

This releases restores zero default standard deviation in the code to
fix an inconsistency with the documentation.

This release includes several improvements:

* sub-yearly (by default weekly) degree-days and melt rate integration,
* temperature variability input via a standard deviation,
* more output variables and custom netCDF output size,
* improved docstrings, command-line help and README.

Initial release.