Enable multi year storage for skipped years

[jacob-mannhardt]

With the new implementation of multi year storage, we currently enforce that years are adjacent, because otherwise we can't model how the storage level behaves over the skipped year.

However, we can just add a term $\Delta L_y$ to the storage level equation in the first time step of a new year that accounts for the delta from the beginning of the previous year to the end, multiplied by the interval between years (for the first year we only have to multiply it by 1, because we only consider the last year as a single year)