Add to_dict method to Metran models

[dbrakenhoff]

to_dict() should store all relevant information to rebuild a model in a dictionary.

[iclectic]

Hi, I would like to have a go at resolving this issue

[dbrakenhoff]

@iclectic thanks for the offer, but we're hosting a development sprint to get new users used to working on github, so I'm saving this issue for that.