Prune saved shifted instances

[getzze]

Following #870, memory is continuously being filled up by saved shifted instances if save_shifted_instances=True (which is always the case if using Flow tracker from the GUI before #974 was merged).

This corrects this bug by pruning the dictionary of reference instances older than track_window before tracking a new frame.

Sorry for not seeing this bug before, if you are using a version between PR #870 and #974, you can have memory issues if you use the GUI and flow tracking.

[codecov]

Codecov Report

Merging #1001 (21fbdf5) into develop (630687b) will increase coverage by 0.01%.
The diff coverage is 90.00%.

@@             Coverage Diff             @@
##           develop    #1001      +/-   ##
===========================================
+ Coverage    66.77%   66.78%   +0.01%     
===========================================
  Files          127      127              
  Lines        21493    21503      +10     
===========================================
+ Hits         14351    14360       +9     
- Misses        7142     7143       +1     

Impacted Files	Coverage Δ
sleap/nn/tracking.py	72.76% <90.00%> (+0.35%)	⬆️

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[roomrys]

Request to add a docstring, but otherwise looks good! Can we add a test for this section of code - I can help write this if you are ready to submit the PR, just let me know.

[getzze]

Thanks for reviewing the commit so fast!
I pushed the changes you proposed.
Regarding the tests, I tried to write them some time ago, but I had a hard time figuring out how to create a mock video and all...