Bugfix in test and data type inference code

[dorisjlee]

Overview

Fixing various bugs and inconsistent logic in data type inference.

• remove unnecessary convert_dtype in float inference (previously added to make integer columns with NaNs detectable as floats, now switch to cardinality cutoff for all)
• bring back cardinality calculation on all float columns (removing the need for special value 999 on float cardinality)
• nominal detection applies even for floats < 20 rows (and associated test changes)

[codecov]

Codecov Report

Merging #349 (2fca0c7) into master (1a72332) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 2fca0c7 differs from pull request most recent head 4ffb277. Consider uploading reports for the commit 4ffb277 to get more accurate results

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
+ Coverage   79.98%   79.99%   +0.01%     
==========================================
  Files          50       50              
  Lines        3612     3609       -3     
==========================================
- Hits         2889     2887       -2     
+ Misses        723      722       -1     

Impacted Files	Coverage Δ
lux/executor/PandasExecutor.py	95.97% <100.00%> (-0.05%)	⬇️
lux/vislib/altair/Heatmap.py	93.10% <0.00%> (ø)
lux/vislib/matplotlib/Heatmap.py	96.66% <0.00%> (ø)
lux/vislib/matplotlib/ScatterChart.py	75.29% <0.00%> (ø)
lux/vislib/altair/ScatterChart.py	96.96% <0.00%> (+3.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1a72332...4ffb277. Read the comment docs.