Meta Issue: Multiple Reactions

[AntoniaBerger]

This issue tracks the implementation of support for multiple reactions per unit and phase.
The goal is to extend the reaction model so that an arbitrary number of reactions can be defined, configured, and computed for each relevant unit and phase.

Todos:

• Add ReactionSystem
• Replace _dynReactionBulk with ReactionSystem in CSTR
• Fix Reaction Test Bug
• Divide reactions module in two cross phase and single phase
• add multiple reactions to FV units
• add multiple reactions to DG units
• add documentation

[jbreue16]

So next are the generalized units, right? :)

[jbreue16]

@AntoniaBerger this can also be closed as per #525 ?