Option to skip saving intermediate products

[vandalt]

It would be nice to be able to skip saving intermediate products from the pipeline. We might even want to make this the default option, given the size that intermediate FITS files can take.

I would see two complementary solutions here:

• Option to skip saving the HDUL for each file (I think this would be fixed automatically by solving #11, as implemented in #17)
• Option to save a "preprocessing" cube, where each HDU is a step of the pipeline. This might be an interesting solution if people want easy access to the different image pre-processing while reducing the output size (the required size is ~ 10% of what it is now from quick tests I did)

[vandalt]

Just wanted to mention I'm working on the first bullet point above. I think it's worth having this available without waiting for #17 to be merged.

EDIT: I actually changed my mind and merged the latest changes in #17 instead.