remove dummy variables from secirvvs model

[reneSchm]

Motivation / Current Behaviour

Since we now use flows in the secirvvs model, we could easily remove all "dummy" variables, as each should be used only for exactly one flow, instead of being added and subtracted to and from one compartment each.

Enhancement description

Move the flow calculation from dummy variables to the flow assignment.

Additional context

"Dummy" is a bad name for a variable (unless we are talking about statistics), if it actually does have a purpose beyond being a placeholder.

Checklist

• [X] Attached labels, especially loc:: or model:: labels.
• [X] Linked to project