Provide contigous spw structures within provided, common and common_good for coarse channels

[gsleap]

For each of: provided, common and common good timesteps/coarse channels; create multiple spws (spectral windows) which then have a vector of coarse channel indices. This will allow callers to have an easy way to get which channels are contiguous.

Structure could be:

struct spw {
coarse_chan_start_index: usize,
coarse_chan_end_index: usize,
metafits_fine_chan_start_index: usize,
metafits_fine_chan_end_index: usize,
bandwidth_hz: f64,
center_frequency_hz: f64, // this is literally the central frequency of the spw and may or may not line up with a fine channel bin
reference_metafits_fine_chan_index: usize, // "reference" is a real fine channel index from the metafits fine chan array and is the  (for odd number of fine chans). For even number of fine chans it is the n/2 bin.
}

There needs to be a public, stand-alone method to get the spw's for an arbitrary input of coarse channel indices. This is in addition to the provided, common and common_good spws.