Faster downsampling, more discrete downsampling warning

[mwouts]

Fixes #113 Fixes #114

[codecov-commenter]

Codecov Report

Merging #118 (84e4e2f) into main (c32000c) will increase coverage by 1.02%. The diff coverage is 93.19%.

@@            Coverage Diff             @@
##             main     #118      +/-   ##
==========================================
+ Coverage   92.73%   93.76%   +1.02%     
==========================================
  Files          19       21       +2     
  Lines         509      625     +116     
==========================================
+ Hits          472      586     +114     
- Misses         37       39       +2     

Impacted Files	Coverage Δ
tests/test_update_samples.py	82.35% <66.66%> (ø)
itables/javascript.py	84.27% <76.19%> (+0.03%)	:arrow_up:
itables/datatables_format.py	84.21% <84.21%> (ø)
itables/downsample.py	98.21% <92.85%> (+0.17%)	:arrow_up:
itables/options.py	100.00% <100.00%> (ø)
itables/sample_dfs.py	100.00% <100.00%> (ø)
itables/utils.py	100.00% <100.00%> (ø)
itables/version.py	100.00% <100.00%> (ø)
tests/test_changelog.py	100.00% <100.00%> (ø)
tests/test_connected_notebook_is_small.py	100.00% <100.00%> (ø)
... and 3 more

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