fix(contours): use nan for mpl contour masks on structured grids

[wpbonelli]

#1615 switched PlotMapView.contour_array() from tricontours to standard Matplotlib contours for structured grids, but did not update the masking mechanism — in the meantime the masked_values argument was ignored for structured grids and only used for vertex/unstructured grids, which still use tricontours.

This PR adds support for structured grid contour masks, converting any masked_values to np.nan before passing the masked array to contour/contourf.

[codecov]

Codecov Report

Merging #1766 (c336ed7) into develop (516721d) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           develop   #1766   +/-   ##
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  Coverage     72.1%   72.1%           
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  Files          253     253           
  Lines        56040   56050   +10     
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+ Hits         40420   40430   +10     
  Misses       15620   15620           

Impacted Files	Coverage Δ
flopy/plot/map.py	83.9% <100.0%> (+0.4%)	⬆️