Implement zscale stretch

[astrofrog]

See this thread for details on how to implement this. If I understand correctly, this should be implemented as an interval class since it should ultimately return a min/max for scaling.

[saimn]

We use an implementation of zscale which can be found at several places (used also in Ginga) but seems to be based on the one of stsci_python/numdisplay:
https://trac.stsci.edu/ssb/stsci_python/browser/stsci_python/trunk/numdisplay/lib/stsci/numdisplay/zscale.py?rev=19347
So I guess this code could be used in Astropy (and moved to an interval class), as numdisplay's license is BSD ?

[astrofrog]

@saimn - yes, since the licenses are compatible that would be an option!

[pllim]

c/c @parejkoj

[parejkoj]

I have an implementation from @ctslater already checked in to the lupton_rgb branch I'm working on (where I needed this):

https://github.com/parejkoj/astropy/blob/luptonRGB/astropy/visualization/lupton_rgb.py#L38

[saimn]

@astrofrog - Can be closed.

[astrofrog]

Thanks!