Merge indicator variables for the subset_selection problem

[pablormier]

Add a method in _Variables to obtain both the raw indicator variables (2 per each positive weighted and reversible reaction, 1 for each negative weighted reaction or positive non-reversible reaction), and the reaction activity (1 if the reaction is active in any direction, 0 otherwise). Return these values with the method get_values() instead of the raw indicators, as they are hard to interpret for users.