Consider separating generic forecast handling

[DamienIrving]

There are a number of packages (see below) that attempt to implement generic forecast handling. We may want to adopt / contribute to one of those packages or write our own xforecast package (using the new xarray flexible indexing; #33) and separate out generic forecast handling functionality from this unseen package, which would just contain functionality specific to UNSEEN likelihood analysis.

https://climpred.readthedocs.io/en/stable/
https://github.com/abkfenris/xarray_fmrc (#38)

[stellema]

• climpred: This looks really useful for importing data, comparing with observations and bias correcting. It might be worth taking a closer look at it?
• xclim: We could contribute any climate indices or bias correction methods that we need to this package.
• xarray_fmc: This might be useful for combining the datasets, but I'm not sure if we need it or if it is actively being contributed to.