Add limb darkening correction

[Cadair]

Maybe based on this: http://hesperia.gsfc.nasa.gov/ssw/gen/idl/solar/darklimb_correct.pro

That calls:

https://darts.isas.jaxa.jp/pub/ssw/gen/idl/solar/darklimb_u.pro
http://hesperia.gsfc.nasa.gov/ssw/gen/idl/solar/darklimb_r.pro
https://darts.isas.jaxa.jp/pub/ssw/gen/idl/solar/darklimb_v.pro

One previous attempt was: sunpy/sunpy#2344

[CyclingNinja]

I'd like to have a crack at this

[CyclingNinja]

https://darts.isas.jaxa.jp/pub/ssw/gen/idl/solar/darklimb_u.pro
http://hesperia.gsfc.nasa.gov/ssw/gen/idl/solar/darklimb_r.pro
https://darts.isas.jaxa.jp/pub/ssw/gen/idl/solar/darklimb_v.pro

calls all the above, Uses a function in Allen's Astronomical Quantities

[abit2]

@Cadair can you please elaborate this would help me understand and make it more clearer.

[Cadair]

@Onoddil you might be interested in this

[nabobalis]

Extra attempt sunpy/sunpy#3728

[dstansby]

Can someone move this across to https://github.com/sunpy/sunkit-image? I think it belongs better there