Interface for CoreFeatures operation

[jmhorcas]

There is also a minor fix in the interface for Configuration to be a list of elements instead of dict.

[jagalindo]

I see some hard limitations when using a list to define configurations. That would prevent us from define negative variability.

[jagalindo]

I see some hard limitations when using a list to define configurations. That would prevent us from define negative variability.

Maybe... Two list and some aux methods?

[jagalindo]

I'd left it as dict by now, as we will integrate operations with negative variability in minf