Change units for "normalize_region" and "continuum_region" requirements for MegaraTwilightFlatImage recipe

[gildepaz]

Currently the units of the regions specified through the "normalize_region" and "continuum_region" requirements for MegaraTwilightFlatImage recipe are in pixels in the processed image. This is not very natural as this regions are intended to select specific sky lines that have fixed wavelengths. At the very least, I would include in the documentation and in the cookbook from which image and how can these limits be obtained.