Update contour line calculations to use ContourPy's LineType.ChunkCombinedNan

[ianthomas23]

Fixes #5957.

ContourPy 1.2.0 includes a new LineType called ChunkCombinedNan that returns the coordinates of multiple contour lines as a single 2D array with nan used as a separator between different lines. This is the format used internally by HoloViews for contour lines, and it is faster and uses less RAM if ContourPy inserts the nan when the contours are calculated rather than HoloViews doing it afterwards.

There is a version check for ContourPy and if >= 1.2 then LineType.ChunkCombinedNan is used, otherwise the original LineType.ChunkCombinedOffset is used and HoloViews inserts the nan. The new complexity is that if we are using datetime coordinates then we cannot pass nan to Matplotlib's num2date function, so here such nan are identified and set to zero using a mask, then num2date is called and the mask used again to set those values back to nan. This is unfortunate but I cannot see a better way that doesn't involve writing a new version of num2date.

Locally tested using both ContourPy 1.1.1 and 1.2.0.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (a3b419a) 88.58% compared to head (2c43d93) 88.43%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
holoviews/operation/element.py	75.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5985      +/-   ##
==========================================
- Coverage   88.58%   88.43%   -0.16%     
==========================================
  Files         315      315              
  Lines       65554    65567      +13     
==========================================
- Hits        58074    57987      -87     
- Misses       7480     7580     +100     

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[ianthomas23]

The reduction in coverage is the contourpy < 1.2 code path that we aren't explicitly testing in CI when we use contourpy >= 1.2.

[hoxbro]

LGTM, thank you for the PR 👍