revert to reversible fixes solution as well

Implements change from #73

Using revert_to_reversible will convert the associated solution object if
it exists to use only the reactions in the resulting model, subtracting
the reverse flux.
pFBA unit test calculates absolute values

It no longer expects the solution.x vector to contain all positive
numbers
Merge pull request #74 from aebrahim/reversible_revert_solution

Reversible revert solution