Validate against user-defined sets #16
Description
In a Calliope model there are particular sets over which all inputs and outputs are built (e.g., timesteps, techs, costs). The set names are consistent across all models, but the contents of these sets are model specific. Therefore, it isn't possible to use the standard ForeignKeys validator.
It would be good have something to be able to validate the content of set columns in all CSVs against a user-defined list of set elements. For example, the sets could be stored in a single CSV (note: they are likely different lengths, so it isn't a 'standard' datapackage CSV) and all other CSVs are validated against the contents of this CSV.
Ideally it would allow for a ForeignKeys style validation: not expecting an exact match, but throwing an error if an unexpected set element is defined in a data CSV.