Write non-SurfaceReactions to CTI files

[francescalb]

Is your feature request related to a problem? Please describe.
I wish to be able to write gasphase reactions in cti-format for cantera/openmkm. As of now it seems to me
that the only reaction type that has this function is the pmutt.omkm.SurfaceReaction.

Describe the solution you'd like
I would like to have a to_cti and to_omkm_yaml functions defined for reaction types that is not SurfaceReaction, most specifically
gas phase reactions for now.

Describe alternatives you've considered
I am not sure which solution is the best, but I can think of two possibilities. 1) to declare an new pmutt.omkm.GasReaction class
that inherits from pmutt.reaction.Reaction and has to_cti and to_omkm_yaml defined.
or alternatively, define them directly in the pmitt.reaction.Reaction class.

Additional context
I am happy to contribute here, but am not sure how you would prefer this done. Also, I might need help for the specification of the output.