Add Metran support to Pastastore

[martinvonk]

I'd like to store / create Metran models in Pastastore such that we can easily do analysis on head series, Pastas simulations (and residuals/noise).

[dbrakenhoff]

This requires metran models to have a to_dict and load method. It would have to store all the necessary internal information to reload the model. There's not much input, but the optimization result will have to be stored. After that it should be fairly easy to store the result with existing write methods...

One issue I'm not sure about is that all of pastastore is written to support pastas models and those methods will not work on metran models. So dealing with that would be quite a challenge.

Anyway, first step is to store/load metran models, so let's do that first.