Implement KPIs disaggregated

[javiarrobas]

This is to implement a get_kpis_disaggregated method at the KPI Calculator that returns the xxxx_dict for each KPI (where xxxx is e.g. ener, cost...). This dictionary is already calculated at the KPI Calculator and stored as an attribute, but has never been exposed to the user. The main motivation for this new feature is to get further insights about the operational performance as this dictionary carries the information of the absolute contribution of each element to the final KPI values. It also relates to DOPTEST (see issue) where such a feature is needed to expose what's the contribution of each energy agent to each KPI at the feeder level.

A first implementation has been made in this draft PR where we were using the name get_kpis_absolute though after some discussion we concluded that get_kpis_disaggregated may be a more representative name of the method.