Support changes in QLP lightcurves in sectors 56+ (renaming of detrended flux columns, etc.) about lightkurve HOT 1 OPEN

QLP changes for extended mission 2 (sector 56+), from Glen Petitpas (with minor edits):

• Now using “DET_FLUX” and “DET_FLUX_ERR” instead of “KSPSAP_FLUX” and “KSPSAP_FLUX_ERR” for the detrended flux and detrended flux error, respectively.
• Now using “DET_FLUX_SML” and “DET_FLUX_LAG” instead of “KSPSAP_FLUX_SML” and “KSPSAP_FLUX_LAG”.
• Introduced a new “SYS_RM_FLUX” which contains the systematics-removed flux light curve (intermediate data product between raw and systematics-corrected+detrended).
• Introduced a new header variable called “DETMET” which provides the name of the detrending method for reference (QSP vs KSP).
• We have moved the QLP flags from bit 13 to bit 31 to prevent any clashes with possible changes to POC flags. Bits 16-29 are unused.
  • bit 13 / 31: Low precision points. The cadence was excluded because low precision
    data occurred in a large fraction of the stars . See QLP data product descriptions

Lightkurve changes needed would be minor:

• Update docstring on the list of possible columns.
• depending on whether it's from sector 56+ or the older one:
  • for the default flux_err_column, use ``det_flux_err, or kspsap_flux_err` or
  • when users choose quality_bitmask="hardest", include bit 31, or bit 13

An existing issue: for quality_bitmasak="hard",

• the current implementation inadvertently includes bit 13 (treating it as the Scattered Light Exclude from SPOC) for QLP.
• For QLP, bit 13 probably either be removed from "hard", or include bit 31 in "hard" as well .

Nuisance in stitching QLP detrended lightcurves across multiple sectors

An user is used to be able to stitch detrended QLP lightcurves across multiple sectors easily:

    # lc_coll : a collection of QLP lightcurves
    lc = lc_coll.stitch(corrector_func=lambda lc: lc.select_flux("kspsap_flux"))

With the new det_flux replacing kspsap_flux in sector 56+, an user would need to handle both cases.

It's unclear whether lightkurve should provide some convenience helper to handle such cases. Possibilities include:

• creating an alias for det_flux as kspsap_flux for new QLP lightcurves (sector 56+). It might shield too much from the users, however.
• Make Lightcurve.select_flux() accept an ordered list of column names. So users can still stitching detrended lightcurves

    # lc_coll : a collection of QLP lightcurves
    # use det_flux column, and if it's not present (for sector 55 and ealier), use kspsap_flux
    lc = lc_coll.stitch(corrector_func=lambda lc: lc.select_flux(["det_flux", "kspsap_flux"]))

from lightkurve.