Support bin_lo/bin_hi as an alternative to bin_edges

[eteq]

As of #645 SpectralAxis's bin edges support only allows continguous bins specified via edges or centers. An enhancement (particularly useful for Xray and other high-energy astronomy - see @eblur's points in #176 and #645 (comment)) is to allow the bins to be specified fully as "lower" and "upper" edges - a common convention here is bin_lo and bin_hi (although @eblur pointed out this might be better as bin_left and bin_right since spectral_axis can go in either "direction"). This allows both overlapping and non-contiguous bins.

The catch here in my mind is that I'm not really sure how to relate this back to the WCS: there may not be a natural way to have non-contiguous bins in either FITS-WCS or gwcs. (maybe @nmearl or @nden can comment?)

There's some further context on this in: #645 (comment), #645 (comment), #494, and #176 (this is basically the sub-set of the discussion in #176 that #645 has not yet addressed)

cc @rosteen