Refactor timeseries

[barneydobson]

Not urgent - but a discussion thread for how data is saved/loaded for WSIMOD models. The current setup is enormously timeconsuming and can make debugging rather hard.

Currently each timeseries is must be converted into the wsimod to_datetime format (which I think is what is taking so long) and stored as keys.

pandas would be the sensible alternative but I avoided it to minimise dependencies - possible it could be optional - and if present used to speed things up a lot.

would probably need to feature in both the model save and load to allow conversion of existing non-pandas models.