Adjust indexing to be same as for content of storage without investment.

This shifts the inital time step to be consistent with a non-invest
storage for models allowing only upfront-invest.

Known issues: For multi-period models, the inconsistent behaviour persists
because the TIMEINDEX does not make a difference between TIMEPOINTS and
TIMESTEPS.

As there is one more tim step, there is one more variable.

The attributes exist for all of the storages in the set.