Model Period as Enum (as opposed to string consts)

[dtrzesniak]

public const int CurrentPeriod = 0;
public const int PreviousPeriod = -1;

could be modelled ad an enum
enum Period { Previous = -1, Current = 0 };
--> I removed the word Period, from the values, as the enum is called Period
--> not sure Period is a good name, as it might clash with something else. To be checked

This would make rendering of parameters reports nicer and also improve understanding of the dictionaries in the storages (int vs Period)