Units cleanup

[cshanahan1]

• Defined a pixel squared unit PIX2 and replaced instances of u.pix * u.pix with PIX2 across the code
• 'locally defined flux units', which is the list of flux units that an input unit is able to be converted to if it is compatible with units in the list with or without u.spectral_density, is now defined in one place only. (I believe the 'create_flux_equivalencies_list' can be further simplified and just use this list as well as the input unit if it is equivalent to a unit in this list, but i did not address that in this PR.)
• some functions added to combine flux and angle strings to get a properly formatted surface brightness unit, used to define the list of all available spectral y axis units
• moved cubeviz unit conversion tests to their own file, out of specviz tests
• added a skipped test that should pass after JDAT 4785 is resolved (@pllim). It was a missing but essential test to test all cubeviz unit conversions that are in the dropdown when a cube is loaded in Jy.