Introduce `oblique_reflectance` calibration

[mraspaud]

(related to #3271 )

As of today, the "reflectance" calibration level refers to both sunz normalized and not normalised "reflectance" quantities. In order to correct the situation, we need to differentiate these two quantities, and we thus need to add a new "oblique_reflectance" calibration level to refer no the non-normalised quantity.

The objective here is to get a minimal working solution, without bells and whistles to start with.

We have identified a few things to discuss/work on:

• Need to add utilities for readers:

if data_id[“calibration”] == _obliquereflectance_name():
    # do oblique-reflectance calibration
elif data_id[“calibration] == “reflectance”:
    # do real-reflectance calibration

• Update reader YAML files with new calibration names where necessary
• Add oblique reflectance calibration name to builtin Enum and any appropriate custom reader YAML definitions for calibration
• Satpy.config entry for using new calibration name for oblique-reflectance
• Do “sunz_corrected” modifiers update the calibration name? Can the Dependency Tree handle that?
  • Would need to test:

scn.load([“C01”], modifiers=(“sunz_corrected”,))
scn.load([DataQuery(name=”C01”, calibration=”reflectance”, modifiers=(“sunz_corrected”,))]) (should this crash?)
scn.load([DataQuery(name=”C01”, calibration=”oblique_reflectance”, modifiers=(“sunz_corrected”,))])

• Lazy workaround: Don’t update calibration in modifier(s)

[simonrp84]

I strongly oppose the use of the term oblique_reflectance for two reasons:

1. This term is rarely used in Earth Observation literature and is never used in any official standards documents (that I can find).
2. This term introduces a source of confusion for sensors with multiangular viewing geometry, for which the off-nadir views are typically referred to as "oblique". S3/SLSTR is the primary example of this confusion, but others exist too.

Instead, I propose use of the term radiance_factor for the non-normalised term, which has previously been used by QA4EO.
I would then suggest reflectance_factor for the normalised term, as this is the correct terminology used by QA4EO and CEOS.

[pnuu]

The oblique_reflectance was my suggestion as the CF standard name for the non-SZA corrected TOA reflectance. The other suggestions tried to explain everything and so are ver,y very, very long.

This can't be radiance_* because it is not radiance, but already divided by the incoming solar radiation and thus unitless.