Code updates to aircraft level calculation and downward RT calculation

[chengdang]

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

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

[chengdang]

All code updates are pushed to branch: https://github.com/JCSDA/CRTMv3/tree/feature/cd_AirCraft_STAR

These updates are merged manually into the current CRTM develop branch, with the following notes:

src/Options/CRTM_Options_Define.f90

• Add new optional flag for downward ADA calculations: Obs_4_downward_P

src/RTSolution/RTV_Define.f90

• A new optional data structure is added to RTV for the downward RT calculation: obs_4_downward_type
• Multiple new internal variables are added, see code block: ... Aircraft level and downward AD calculation

src/RTSolution/Common_RTSolution.f90

• Add assignment of the downwelling radiance if RTV%obs_4_downward%rt = TRUE

src/RTSolution/ADA/ADA_Module.f90

• Add downward RT calculations of ADA solver in SUBROUTINE CRTM_ADA

src/CRTM_Forward_Module.f90

• Add downward radiance calculation: IF ( Opt%Obs_4_downward_P > ZERO ) THEN

• The following code block is moved to a new n_channel_threads loop:

      ! Process aircraft pressure altitude
       IF ( Opt%Aircraft_Pressure > ZERO ) THEN
          RTV(nt)%aircraft%rt = .TRUE.
          RTV(nt)%aircraft%idx = CRTM_Get_PressureLevelIdx(Atm, Opt%Aircraft_Pressure)
          ! ...Issue warning if profile level is TOO different from flight level
          IF ( ABS(Atm%Level_Pressure(RTV(nt)%aircraft%idx)-Opt%Aircraft_Pressure) > AIRCRAFT_PRESSURE_THRESHOLD ) THEN
             WRITE( Message,'("Difference between aircraft pressure level (",es22.15,&
                  &"hPa) and closest input profile level (",es22.15,&
                  &"hPa) is larger than recommended (",f4.1,"hPa) for profile #",i0)') &
                  Opt%Aircraft_Pressure, Atm%Level_Pressure(RTV(nt)%aircraft%idx), &
                  AIRCRAFT_PRESSURE_THRESHOLD, m
             CALL Display_Message( ROUTINE_NAME, Message, WARNING )
          END IF
       ELSE
          RTV(nt)%aircraft%rt = .FALSE.
       END IF

[chengdang]

Note that all the updates in https://github.com/JCSDA/CRTMv3/tree/CloudRadar_AirCraft regarding radar sensor has not been made, therefore no changes to the subroutine CRTM_Compute_CloudScatter