Add centroid finder block

[davidasamy]

Finds centroid of instance -> #3

[codecov]

Codecov Report

Merging #7 (28b8f6d) into main (5903df4) will increase coverage by 6.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #7      +/-   ##
==========================================
+ Coverage   90.00%   96.07%   +6.07%     
==========================================
  Files           3        4       +1     
  Lines          20       51      +31     
==========================================
+ Hits           18       49      +31     
  Misses          2        2              

Impacted Files	Coverage Δ
sleap_nn/data/instance_centroids.py	100.00% <100.00%> (ø)
sleap_nn/data/providers.py	100.00% <100.00%> (ø)

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[davidasamy]

Address:

• Currently each sample read through LabelsReader is a singular instance and corresponding frame. Change this to a list of instances and a corresponding frame instead?
• Currently centroids are always found through the bounding box midpoint find_points_bbox_midpoint, must also add option to specify anchor node.