beams returns array of arbitrary length

[keflavich]

The beams returned with .beams can be smaller than the spectral axis shape, which probably isn't what we want:

spectral-cube/spectral_cube/base_class.py

Lines 465 to 467 in 606b1ef

	@property
	def beams(self):
	return self._beams[self.goodbeams_mask]

I think we should instead be returning a masked array. Any thoughts on how we should do this? Is the Beams object actually compatible with being masked in that way?

[e-koch]

Not currently, but a masked array seems necessary for this reason.