Irreversible reaction taken into account

The irreversible reactions are taken into account, using a boolean and testing at the calculation step.

Everything is happening in the Reaction object, the calculation of rate of progress are changed into the following pattern:

1 - calculate the forward component to the rate of progress and derivatives,
2 - check if the reaction is reversible, if no, return at that point
3 - if yes, calculate and add the backward component.

A test has been added, the existing tests has been adapted (reversibility always assumed), the parsing/initializing steps have been updated also.