Refine timeindex

[p-snft]

As different types of periods will be used for multi-period optimisation (long-term investment planing) and working with aggregated time-series (e.g. using tsam), I suggest to change the indexing of the flows:

• flows have index [i, o, t] (types Node, Node , int), we use TIMESTEPS for t. Refine time index of Flows #1077
• Storage content should use TIMEPOINTS
• The model has a lookup function to find out which period(s) a time step belongs to:
  • Multi-period: Each time step belongs to one period. Limits can be a function of the period.
  • Aggregation: Each time step can belong to multiple periods.
• TIMEINDEX [int, int] for period, timestep is removed