Issue with plot labels

[williamjameshandley]

Description

#207 introduced a bug whereby samples.plot_2d(...) no longer has the correct tex in all of the labels. This arises because samples.plot(...) defaults to using the column label, which can overwrite the labels set up by make_2d_axes. This fixes that by passing xlabel explicitly through samples.plot in samples.plot_2d(...)

Checklist:

• I have performed a self-review of my own code
• My code is PEP8 compliant (flake8 anesthetic tests)
• My code contains compliant docstrings (pydocstyle --convention=numpy anesthetic)
• New and existing unit tests pass locally with my changes (python -m pytest)
• I have added tests that prove my fix is effective or that my feature works

[codecov]

Codecov Report

Merging #218 (0a1b54c) into master (46a716d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   96.61%   96.62%   +0.01%     
==========================================
  Files          25       25              
  Lines        1947     1954       +7     
==========================================
+ Hits         1881     1888       +7     
  Misses         66       66              

Impacted Files	Coverage Δ
anesthetic/samples.py	100.00% <100.00%> (ø)

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[lukashergt]

Nice quick bug catch.