Add utility functions for extracting useful information about an ensemble

[gregmedlock]

Some utility functions that have come up as potentially useful so far:

• getting a list of all reactions that vary across an ensemble, rather than iterating over all features and getting base_components
• getting all unique states for a given feature or for a base_component associated with multiple features (e.g. is the upper bound for a feature always either 0 or 1000? are the bounds for a reaction always (0,0) or (1000,1000)?)

This issue will be a running list for some of these utility functions.

[gregmedlock]

• Add a utility function that counts the number of reactions and metabolites in a member.