request: support averaging in fits2fits, not just binning

[o-smirnov]

I've realized the "bin" option in fits2fits actually performs as advertised, i.e. sums the pixels in a bin. Would it be possible to add an option to take the mean value instead of a sum?

We usually deal with flux density images, so summing breaks our units...

[ericmandel]

Yes, I can do that (not in one day, though) ... must be nice to have so many photons that you need to take the mean (not the X-ray experience!)

[ericmandel]

JS9 now supports averaging the pixels as well as summing them when binning. You can:

1. set JS9.globalOpts.binMode to "a" (averaging) or "s" (summing) in your js9prefs.js file to change the global default.

2. use the Preferences plugin to change the value on the fly for that session.

3. append an "a" to the bin value in the Binning plugin to set "averaging" on the fly for that section.

4. JS9.Preload("foo.fits", {bin: "4a"}) to load a binned file using averaging.

It's all updated in GitHub. Lots of changes to offer all of the above options, so let me know if you find a problem.