Add contourf method to axes

[eldond]

Description:

• close Add contourf (filled contour) method #51

Pre-merge checklist (in addition to automatic checks):

• Author has reviewed and finalized code
• New functions, classes, and keywords are described in docstrings
• This pull request is ready for review / merge

[eldond]

It almost works:

[eldond]

[codecov]

Codecov Report

Merging #52 into dev will decrease coverage by 1.3%.
The diff coverage is 86.32%.

@@            Coverage Diff             @@
##              dev      #52      +/-   ##
==========================================
- Coverage   98.77%   97.46%   -1.31%     
==========================================
  Files          23       23              
  Lines        1795     2015     +220     
==========================================
+ Hits         1773     1964     +191     
- Misses         22       51      +29

Impacted Files	Coverage Δ
tests/test_axes.py	99.36% <100%> (ø)	⬆️
tests/test_contour.py	99.18% <100%> (+1.06%)	⬆️
tests/examples.py	91.66% <12.5%> (-4.28%)	⬇️
pgmpl/contour.py	88.54% <83.07%> (-11.46%)	⬇️
tests/test_tracking.py	96.66% <0%> (-1.07%)	⬇️
pgmpl/tracking.py	100% <0%> (ø)	⬆️
pgmpl/axes.py	100% <0%> (+0.29%)	⬆️

---

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[codecov]

Codecov Report

Merging #52 into dev will decrease coverage by 1.30%.
The diff coverage is 86.69%.

@@            Coverage Diff             @@
##              dev      #52      +/-   ##
==========================================
- Coverage   98.78%   97.47%   -1.31%     
==========================================
  Files          23       23              
  Lines        1804     2021     +217     
==========================================
+ Hits         1782     1970     +188     
- Misses         22       51      +29     

Impacted Files	Coverage Δ
tests/examples.py	91.66% <12.50%> (-4.28%)	⬇️
pgmpl/contour.py	88.88% <83.82%> (-11.12%)	⬇️
tests/test_axes.py	99.36% <100.00%> (+<0.01%)	⬆️
tests/test_contour.py	99.18% <100.00%> (+1.06%)	⬆️
tests/test_tracking.py	96.66% <0.00%> (-1.34%)	⬇️
pgmpl/axes.py	100.00% <0.00%> (+0.29%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[eldond]

[eldond]

https://pyqtgraph.readthedocs.io/en/latest/3dgraphics/glsurfaceplotitem.html ?