Add ability to only register frames with a minimum number of star pairs

Hello!

I've been using Siril with noisy data, and it's been failing to normalize some light frames. I've tracked this down to poor registration of some of my bad frames, and came up with a solution to automatically weed them out.

This MR adds an option to global star alignment registration, where you can specify a minimum number of star pairs with the reference frame before a frame is registered.

For example, this command will only register frames with 80 or more star pairs with the reference frame:

register pp_light -minpairs=80

This works because bad frames have a small number of star pairs (~20), while a good frame will have many more (100's to 1000's).

I have not been able to wrangle the GTK interface to add a configuration option, if someone wants to help out with that it would be greatly appreciated!