Separate extinction correction from sensitivity function

[rcooke-ast]

When combining standard star exposures, the extinction correction should be applied on each individual observation. A note that was introduced in PR #1770:

The extinction correction cannot currently be done in the datacube creation because the sensitivity function algorithms correct the standard star for extinction when generating the sensitivity function. Including the extinction correction in the datacube would result in a double correction of the standard star for extinction. This could be wrong when combining multiple standard star exposures if the airmass of the standard star exposures is significantly different. For now, to stay consistent with the current pipeline, the extinction correction is done in the sensitivity function algorithms, with the caveat that the standard star exposures are assumed to have similar airmasses. For now, comment out the extinction correction, and reapply this later when the sensitivity function algorithms are unified.