Atlas filtering based on estimated dispersion

[jveitchmichaelis]

Currently we just extract all atlas lines between two wavelengths. While it's useful to store all catalogue lines, it can confuse the algorithm:

• if line separation is lower than the resolution of the instrument
• during preliminary peak/arc line matching there is a higher uncertainty in matching multiple close atlas lines to a single peak (particularly if only one peak in a group is extracted)

We can't necessarily filter by relative intensity because this is instrument specific and small peaks at the edge of the spectrum can be critical for accurate fitting.

Proposal:

• Optional (default true) ignore atlas lines which are closer together than the lowest possible resolution we expect (e.g. ((end_wavelength + tolerance) - (start_wavelength - tolerance))/n_pix)
• Once a good coarse calibration has been obtained, fit again using much stricter matching criteria (could repeat this until the final set of matches doesn't change).
• Could initially use high relative intensity lines + peaks for RANSAC, and then relax this criteria for later fitting.

[cylammarco]

Also refer to #16