Display IFU header

[leonardoubeda]

It would be very helpful to have the option to display the contents of the IFU header in a separate window, at least for the science extension.

[astrofrog]

I can work on this, I need to implement it in glue anyway (glue-viz/glue#1521)

[astrofrog]

This now exists in glue - control click on a dataset/subset to see an option to view the header. Do we want to provide a more obvious way to do this in cubeviz?

[brechmos-stsci]

This is now incorporated.