Implement the POINT strategy in the engine

[micheles]

POINT = Propagation Of epIstemic uNcerTainty is a new methodology invented by Marco Pagani to manage single site calculations with few sources and a complex logic tree. We need to implement it in the engine, one step at the time.

NB: the current prototype works by parsing the outputs as CSV files, while instead I want to read the outputs
directly for the datastore. Also, I want to move from using multiple calculations and multiple datastores to using
a single calculation.