Set peakfinder min_sep based on bin width(s)

[jvavrek]

From #263, it would be useful to set the min_sep in the peak finder based on the bin width. The complication here is accounting for non-uniform bins—we'll need to look up the bin width of a prospective peak, rather than just use e.g. spec.bin_widths_raw.min()

[jvavrek]

Could handle the uniform case automatically, and warn/error if the bins are non-uniform.