Simplify reaction parameters interface

[schmoelder]

Currently there is some ambiguity regarding the parameter fields of reactions.

Sometimes, the field reaction is used for the bulk parameters, sometimes reaction_bulk.

I suggest the following approach:

Always distinguish between reaction_model_bulk, and reaction_model_particle. When a unit has no particles, ignore reaction_model_particle. Consequently, for the parameters, always use reaction_bulk and reaction_particle respectively.

See also this discussion.