essential_genes and essential_reactions extra arguments and outputs

[EmanuelGoncalves]

Hi,

It would be great to provide an extra argument to the essential_genes and essential_reactions functions, for example elements, that would define the list of elements to be tested. This would avoid testing all genes/reactions in a model if we are only interested in a subset.

Additionally, instead of just returning a list of essential genes/reactions it would be useful to return an object, e.g. pandas.DataFrame, with the solution objective values of all tested elements. This way other types of min_growth thresholds could be applied without having to rerun the functions.

I understand these are convenience functions and probably lower level functions could be useful. If so, then I think it could be worth add these to the docs.

Thanks a lot,