Refactor parallelization of bias calculations

[castelao]

Why this feature is necessary:

From discussions on #215, @bnb32 suggested to register as a ToDo so we don't forget about it.

A possible solution is:

Different possibilities:

• Combine all the redundant code on parallelization with futures;
• Move to xarray and reduce a lot of code while parallelizing with Dask;

I have considered the following alternatives:

For now it is just redundant and explicit so it is prepared for an easy and clean refactoring.

Additional context

None

Charge code

Urgency / Timeframe

Not urgent. It works as it is.