Add clean scales as ratios of the beam size

[e-koch]

Instead of giving clean scales only in arcsec:

interf_config   	  12m       {'clean_scales_arcsec':[0.5,1,2.5,5.0]}

enable a mode in Nx the beam:

interf_config   	  12m       {'clean_scales_beam_ratio':[0, 1.0, 2.0, 5.0]}

This will be more useful for some of the higher-res CO(2-1) mapping that use different 12m configs to reach the same physical scales across the sample.

[thomaswilliamsastro]

might also suggest you could keep the variable name but just distinguish between floats and strings, so then interpret 0.5 as in arcsec but "0.5 * beam" (or whatever) as half the FWHM. Might open things up to using astropy units more generally