feat: sparse horizon handler for vdf

[sebhoerl]

The current horizon-based handler for VDF aggregates flow over N iterations for every link in every interval (for example, 5 minutes). All of this is saved as one huge block in memory and also generates large files in the output when running large-scale scenarios, for instance when using the whole network of Bavaria.

This PR introduces (experimentally) the "Sparse Horizon" handler, which works exactly as the standard version, but only saves observations that have actually taken place. So if a link in a specific time step in a specific iteration had zero flow, it will not appear in memory. This will very likely drastically decrease memory use for large-network simulations which are run with low population sampling rates.

TODO;

• Verify that this generates the same output as the standard Horizon handler
• Verify that reading and writing works correctly