Make 'units' in lineplot functional

[mwaskom]

This PR adds some missing functionality from #1285. It makes the unit parameter functional for drawing replicate lines with the same semantics:

fmri = sns.load_dataset("fmri").query("region == 'parietal'")
ax = sns.lineplot(x="timepoint", y="signal", hue="event", units="subject",
                  estimator=None, linewidth=1, data=fmri)

This overloads the "units" parameter that is used elsewhere to do a hierarchical bootstrap. The hierarchical bootstrap is not currently implemented in lineplot. I think this makes sense because the parameter picks out the same dataset element in both cases. For the time being we're going to force specification of estimator=None when using units.

TODO

• Raise when we have units and estimator
• Tests
• API example

[codecov]

Codecov Report

Merging #1433 into master will decrease coverage by 0.01%.
The diff coverage is 93.93%.

@@            Coverage Diff             @@
##           master    #1433      +/-   ##
==========================================
- Coverage   94.19%   94.17%   -0.02%     
==========================================
  Files          15       15              
  Lines        4220     4240      +20     
==========================================
+ Hits         3975     3993      +18     
- Misses        245      247       +2

Impacted Files	Coverage Δ
seaborn/basic.py	97.78% <93.93%> (-0.41%)	⬇️

---

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 c5be9bc...4ceb48e. Read the comment docs.