Move adsorption/reaction type to their respective branch (indead of unit operation level)

[schmoelder]

I noticed the unit_type is specified within the branch of the unit itself. However, for the adsorption models and reaction models, the model is not defined on the same level as the parameters, but also on ‘unit-level’. It does not make a huge difference, but this might be also be worth considering. Certainly makes modular changes of the models easier, since only one branch needs to be exchanged instead of the parameter branch and the model field.

Unit operations (for reference):

cadet.root.input.model.unit_001.unit_type = 'CSTR'
cadet.root.input.model.unit_001.ncomp = n_comp
...

Adsorption/Reactions (current):

cadet.root.input.model.unit_001.adsorption_model = 'LINEAR'
cadet.root.input.model.unit_001.adsorption.LIN_KD = n_comp*[1]
...

cadet.root.input.model.unit_001.reaction_model_bulk = 'MASS_ACTION_LAW'
cadet.root.input.model.unit_001.reaction_bulk.stoich = [...]
...

Adsorption/Reactions (proposed):

cadet.root.input.model.unit_001.adsorption.adsorption_model = 'LINEAR'
cadet.root.input.model.unit_001.adsorption.LIN_KD = n_comp*[1]
...
cadet.root.input.model.unit_001.reaction_bulk.reaction_model_bulk = 'MASS_ACTION_LAW'
cadet.root.input.model.unit_001.reaction_bulk.stoich = [...]
...

In order to guarantee backwards compatibility, this could be implemented by either checking both locations and/or by implementing/fixing #18 .