More choice for input SEDs

[nmiller95]

Will try to include the following SED catalogues in a future update:

• Coelho high resolution models (limited wavelength coverage; might work with some wrangling)
• Kurucz ODFNEW models (good parameter and wavelength coverage; less dense in wavelength space but may be ok)
• Kesseli models (same problem as Coelho)

Might also be a fun challenge to give the option to restrict the BT-Settl wavelength range to see how the distortion polynomials handle it, since the other models also require a different wavelength range to that currently in use.