[MPM] Compute reactions flag

[jginternational]

Hi @ncrescenzio

Please change this draft into a PR when you feel that this changes can fit into MPM applications in production

[lauramoremar]

Hi! I think we should include this option in the interface, since it is useful when reactions are requested for plotting in the postprocess or for computing forces into a file. My only doubt is whether this is the correct section Strategy parameters in the interface, or if it should instead belong to the postprocess section, Results (although, at the same time, it is also something that needs to be computed...).

What do you think? @ncrescenzio @andimkatili @antonialarese

[ncrescenzio]

@lauramoremar I would leave it here because it is more consistent with the structure of the json parameters. Moreover this option is also used to initialize the MPMResidualBasedNewtonRaphsonStrategy, so I think it makes sense to leave it here