New utility function in Solution class: show_metabolite_balance
Removed presolve in gurobi. In the future all configurations should be centralized in a config file.
Merge https://github.com/cdanielmachado/framed into marta-branch
tolerance related fixes

the tolerances in gapFill, gapFind, and blocked_reactions were changed
to 1e-5.
these are needed when integer variables actually have values slightly
different than what's being tested for