Update structure of functions to save files

[altuson]

This PR makes several updates to the functionality that saves the TPF and LCF.

Changes made:

• Updated APERTURE image HDU in the TPF. Instead of saving the median aperture over time, this now saves the flattened aperture i.e. any pixel that is included in the time-dependent aperture at least once.
• Added flag 13 to LC quality - mask had 2+ discrete regions. This can happen in the case of a fast-moving target with a high threshold for inclusion in the mask.
• Updated _aperture_photometry() and _psf_photometry() to return dictionaries instead of arrays.
• Added asymmetric error bars to the magnitude light curve.
• Added an EXTRAS extension to the LCF. This includes extra information such as CORNER1/2, ORIGINAL_TIME, RA_PRED/DEC_PRED.
• _make_tpf_hdulist() and _make_lc_hdulist() now return an HDUList rather than an HDU.
• Updated and re-organised keywords in headers of all extensions in the TPF/LCF.
• Added option to save diagnostic plots from the scattered light and background model.
• Added observing geometry parameters (distance from Sun, distance from TESS and Sun-Target-Observer angle) into LCF EXTRAS extension. Also added average values to primary header of TPF and LCF.
• Created utils function add_column_header_comments() to add comments to FITS column header keywords (e.g. TTYPE, TFORM, TUNIT) in a binary table HDU.
• Updated README file.
• Updated tutorial notebooks 1 & 2.
• Created tutorial notebook 3 to demonstrate creation of light curves using multiple apertures.
• Updated existing tests in test_movingtpf.py.
• Added new test to test_movingtpf.py for creating multiple apertures.
• Created new file test_utils.py for testing utils functions.
• Bumped version number.
• Updated required version of tess-ephem.

[jorgemarpa]

So far, this is good. I left you a couple of small coments