Account for past observations

[TomWagg]

Currently the way I have it set up is that the algorithm doesn't account for any observations prior to the detection window. This leaves us with 3 cases

1. It hasn't been observed before
2. It has been observed at least twice on a previous night but not enough to be detected
3. It's been previously detected

For 1, we proceed as usual. For 3, we should remove these objects from the run since we've already found them.

2 is more complicated but I propose we do the following. For each object, pass a list/dict of nights on which 2 observations have occurred that meet the usual criteria. We can then prepend this to unique_nights in get_probability_by_id and run it in exactly the same way.