new devel release improves handling of CDF files

[Jeremy]

I've made an update to the devel release of Autoplot, which reworks its use of the CDF library and should now be able to read and write arbitrarily large CDF files, as long as each variable is less than 2GB.  This impacts quite a bit of code, and you would be helping me tremendously if you tried this version and verified that you can still operate with data in CDF files.

Other changes include handling "$x" fields in the pngwalk tool by showing the date and matched content with each image, and cleaning up the CDF GUI so that the special X and Y selections don't include datasets which won't work.

Let me know how this release works for you!  This is the development release at http://autoplot.org/jnlp/devel/ (20220924a).

Jeremy