Add functionality to extract the simplified graph from the CityTemporalTiler

[DiegoVinasco]

Add a --extract-simplified-graph option to the temporal tiler to output the simplified graph as JSON before tiling.

[EricBoix]

The code that reconstructs the temporal graph CityTemporalTiler::main() happens at a pre-computation stage of the temporal tiler and is independent from it. If the need is to extract that intermediate object, maybe it would be better to separate this concern/goal from the TemporalTiler and make it another independent programs ? (while respecting DRY and having the two codes share that pre-computation function)