Add heatmap of the zoom window to the plotattributes

[felixcremer]

This allows to use the inner heatmap plot for showing the colorbar.
This is a workaround, but at least this allows the following:

julia> fig, ax, hmap =heatmap(pyr_ships["1"])

julia> Colorbar(fig[1,2], hmap.__pyramid_heatmap[])
Colorbar()

[felixcremer]

@asinghvi17 maybe you would have a good idea how to better integrate it, so that Colorbar(fig[1,2], hmap) would work?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.10%. Comparing base (2ff01a7) to head (2799a3b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   71.77%   72.10%   +0.33%     
==========================================
  Files           4        4              
  Lines         333      337       +4     
==========================================
+ Hits          239      243       +4     
  Misses         94       94              

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[felixcremer]

Needs test, that the Colorbar works.