Change default behaviour of SunZenithCorrector/sunz_corrected

[strandgren]

With #3397 it is suggested that the default behaviour of sunz_corrected modifier (SunZenithCorrector) should be changed to compute the true reflectance in Satpy >= 1.0. We would then have the sunz_corrected modifier that computes the true reflectance for scientific or quantitative use and the effective_solar_pathlength_corrected modifier which uses the Li and Shibata (2006) parameterization to avoid over-correction at high Sun zenith angels for (RGB) imagery.

For now, #3397 keeps the current default values and behavior (reduction of the sunz correction between 88-95 degrees), but adds a config parameter that can be set to either retain the legacy default behavior of the SunZenithCorrector/sunz_corrected modifier, also after upgrade to Satpy v1.0 or to opt in for the new behavior (no reduction and simple 1/cos(sunz) correction) already before Satpy v1.0. This can be controlled set with:

import satpy
satpy.config.set(use_legacy_sunz_correction=True)  # or False

With #3397 use_legacy_sunz_correction is set to True by default, making sure to keep the current/legacy behavior. Hence, we simply have to set the defautl configuration value it False in Satpy v1.0 to have a direct switch to the new behaviour of SunZenithCorrector and sunz_corrected

Later, once the changes in #3397 and v1.0 have been established (say for Satpy v1.1), we can remove the option to use the legacy behaviour and cleanup the code accordingly:

• Remove use_legacy_sunz_correction in _config.py
• Change default values for correction_limit and max_sunz of SunZenithCorrector to None
• Cleanup init docstring in SunZenithCorrector
• Remove handling of __default__ and self.use_legacy as well as warning in SunZenithCorrector._apply_correction()
• Remove deprecated tests covering both ew and legacy behaviour and adapt expected output for testing default behaviour of SunZenithCorrector (see f3b605d)
• Remove deprecated class init input arguments and in EffectiveSolarPathLengthCorrector
• Remove warning in EffectiveSolarPathLengthCorrector about deprecated input arguments.
• Remove tests for warnings