ginga-based 1D spectrum viewer: Redshift

[kbwestfall]

Enable pypeit_show_1dspec --ginga to specify the redshift of a spectrum by placing the mouse on a feature with a known rest wavelength.

[ejeschke]

@kbwestfall, what is the keystroke in linetools that does this? Probably I can add it to #2041 or in a follow up PR

[ejeschke]

I don't think this one is implemented yet. Prematurely closed?