Add pickle support

[snorfalorpagus]

It should be easy to add pickle support to the classes. This could be tied into read/write support for JSON.

(I realise catchment_from_json() exists - I was thinking, Catchment.load() and Catchment.dump().

[jetuk]

Would load and dump use a JSON format?

[snorfalorpagus]

Yes. I was thinking that load and dump handle the dictionary of parameters. The __setstate__ and __getstate__ methods could then make use of this, plus a little extra to handle the actual state.