Allow pMuTT to parse reaction strings with multiple reaction delimiters

[wittregr]

Currently pMuTT only allows one reaction delimiter when parsing reaction strings. There are time when both reverisble and irreverisble reactions might be required. This would take more than one reaction delimiter. For example, if a reversible reaction were designated by '=' or '<=>' then an irreversible reaction might require a '=>' reaction delimiter.