SIMOBS-82: Aircraft-level sensors and downward ADA calculation

[chengdang]

This epic contains the efforts to create or improve the following CRTM features:

• Implement downward adding-doubling calculation, for CRTM simulations/DA of ground-based and aircraft-level sensors.
• Improve CRTM aircraft level calculations, including forward, TL, AD, and K_matrix modules.

The initial code updates are delivered by Mark and Yingtao, in branch: https://github.com/JCSDA/CRTMv3/tree/CloudRadar_AirCraft

This epic focuses only on the calculations regarding aircraft level sensors and the radiative transfer solver.