Quick sky update

[ajmejia]

Adding sky extensions for master sky and super sampled sky. Main changes are:

• Implemented RSS._sky and RSS._sky_error attributes
• Implemented Spectrum1D._sky and Spectrum1D._sky_error attributes
• Added SKY and SKY_ERROR extensions in RSS outputs
• Added optional sky weights in quick reduction and in outputs metadata
• Implemented optional sky subtraction (super sampled and master skies are still saved)