Add CISTA-LSTC and CISTA-EIFlow

[lsying009]

1. Add CISTA-LSTC and CISTA-EIFlow
2. CISTA-EIFlow also output flow estimation, add save flow
3. Add 'low_k_events' evaluation mode: If the target number of events per reconstruction is $k$, We either divide events between two consecutive frames by $k$ into multiple groups or combine events across several frames into a single group for the reconstruction and flow estimation. Related configuration is in config/eval/low_k_events.json.
4. Add PSNR metric
5. Add normalization for ECD dataset