Update time-based algorithms to use TimeIndexEntry

[arienandalibi]

Now that many of the APIs use TimeIndexEntry instead of timestamps, it might be time to update the epidemics temporal_SEIR algorithm to use TimeIndexEntry instead of timestamps. We are currently extracting the timestamp:

The Temporal Reachability algorithm (temporally_reachable_nodes) should also be updated to use TimeIndexEntry.

This may apply to other time-based algorithms as well.