Eliminate the extra timestep

[FBumann]

The extra tilmestep needed for certain constraints/variables/input parameters is quite unpleasant.
The following would benefit greatly from its removal:

• Access to the solution of variables (no nan fillers)
• Simplified index handling
• Simplified parameterization (all arrays of same size), and less confusion

Did we evaluate all options to handle this?
@baumbude @PStange

It might be sufficient to handle the final charge state as an extra variable with no index.

[FBumann]

Closed somewhere in #348