Parallelism with cython.parallel

[snorfalorpagus]

The indepedent areas that comprise a catchment in catchmod lend themselves nicely to parallelisation.

http://docs.cython.org/src/userguide/parallelism.html

[jetuk]

It is probably better to do this across the multiple runs in a single Catchment.step rather than the subcatchments.

[snorfalorpagus]

We could make it an option to do either? I still think common use is only one scenario.