Don't use offset / scientific notation in numeric legends

[mwaskom]

Fixes #3174:

This takes the shortest path solution of simply disabling the offset / scientific notation on a scalar formatter used in the legend. I think it's possible that we'll want to add those as options in Continuous.label in which case we may want to figure out how to show the extra information in a legend context (e.g. as a subtitle or extra row at the bottom of the legend). Punting on that as it seems complicated.

Note that if you do want to format legend entries with scientific notation, you can do Continuous().label(like="{x:.2g}") (not exactly the same thing, but will help when variables have very large values).

[codecov]

Codecov Report

Merging #3187 (9372112) into master (22cdfb0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3187   +/-   ##
=======================================
  Coverage   98.41%   98.41%           
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  Files          76       76           
  Lines       24093    24110   +17     
=======================================
+ Hits        23712    23729   +17     
  Misses        381      381           

Impacted Files	Coverage Δ
seaborn/_core/scales.py	89.86% <100.00%> (+0.09%)	⬆️
seaborn/utils.py	95.49% <100.00%> (+0.02%)	⬆️
tests/_core/test_plot.py	98.68% <100.00%> (+<0.01%)	⬆️
tests/test_relational.py	99.31% <100.00%> (+<0.01%)	⬆️