fluxanalysis: Add functions to check for spurious production/consumption

[jonls]

Adds two functions that check for spurious production and for spurious consumption, respectively. For example, the reactions A => B + C and C => A allow for a spurious production of compound B. This check is complementary to the stoichiometric check because the stoichiometric check reports on potential model problems whereas the spurious loop check reports on the subset of stoichiometric problems that actually influence the simulation results.

Also adds the option to do psamm-model masscheck --type=loop which will detect and report all production/consumption loops.