TL implementation for aircraft/user-defined level upward/downward

[chengdang]

Description

This PR contains:

• TL implementation for aircraft/user-defined level upward/downward radiance calculation, see module CRTM_ADA_TL
• Updates to code format for module CRTM_ADA
• New notes and comments in ADA_Module.f90
• New ctests for aircraft/downward modules

Issue(s) addressed

#182

Dependencies

None

Impact

TL results of aircraft level calculation are updated.

Checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have run the unit tests before creating the PR

[chengdang]

@quanhualiu, hello Mark, as we discussed, this is the TL implementation for the updated aircraft level calculation.
Could you please review the implementation, especially the new equations in module CRTM_ADA_TL. Thank you!

[StegmannJCSDA]

@chengdang Thank you for including me in the review. I just have a few comments so far:

• Are there any test results that highlight the impact of the changes in the Forward module?
• Do the changes for the aircraft mode debugging and the downwelling radiance overlap, or could they have been split up in two PRs?
• Did you use TAPENADE to derive the TL code, or by hand?
• For the regression tests, the Load_Atm_Data.inc file etc. is repeated in a lot of folders now. Would it make sense to move it down at this point, e.g. to mains/regression?