Support for np.nan background_color

[nkemnitz]

Updates CloudVolume (see seung-lab/cloud-volume#641)

Allows cue specs with NaN support... kinda:

{
	"@type": "build_cv_layer"
	"path":   "..."
	"cv_kwargs": {
		"background_color": { "@type": "np.nan"}
	}
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (57cf04c) to head (304cbc2).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #810   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          142       142           
  Lines         5945      5947    +2     
=========================================
+ Hits          5945      5947    +2     

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