DM-39243: Add a factory method for OwnedImagePlanes

[arunkannawadi]

This adds a convenient factory method to generate OwnedImagePlanes.

[TallJimbo]

This doesn't actually use anything from the Exposure that isn't part of MaskedImage. Better to call this from_masked_image and just take MaskedImage.

[arunkannawadi]

True, but I was hoping to avoid the .getMaskedImage() call and pass the Exposure object instead (which would still run). Is your preference for this also driven by your desire to stop using Exposure whenver possible?

[TallJimbo]

I do want to reduce usage of Exposure when it's not adding much on top of MaskedImage, because I'd like to someday diminish its use to the point where we could drop it.

But even if we had no intention of dropping it ever, it'd still be inappropriate to use it in places where it's adding nothing to MaskedImage, as is the case here and in the drp_tasks branch; this is the Interface Segregation Principle. It's the same reason AccumulatorMeanStack only deals with MaskedImage, not Exposure.

(Yes, the drp_tasks branch does call getWcs, but it also has that same WCS throughout the exposure's lifetime already in the local skyInfo variable, so it doesn't have to.)