Support saving plots to memory file objects

[karlmsmith]

From GFDL (John K.) -
Enable figures produced in pyferret to be written to a memory object that can be served directly from, e.g., a web server (matplotlib does this). This would only apply to no-display mode - does not make sense to do this when displaying plots - and probably only with PNG images. The Cairographics library enables hooking in your own function for writing PNG output. Such a function could be write the desired memory object.

This would streamline incorporation of plots in Jupyter notebooks.

[eugeneburger]

@josborne-noaa FYI - have you considered this?

[seasage]

I'll look into it. Seems very doable.