Save/load metran models

[dbrakenhoff]

It would be nice to build a pas-like file (see Pastas) to save models and load them again.

We could try to follow pastas' style and have the different relevant objects in a model contain to_dict() methods. Similar to pastas this would include a series keyword argument to store the dictionary with or without time series.

We can reuse the pastas encoder to write certain datatypes to json for storing it as a file. I guess we could use a .metran extension or something similar.