Create unit tests for aircraft level/downward ADA calculations

[chengdang]

Create unit tests for new features listed in issue #157.

[chengdang]

To output the downwelling radiance at pressure P, set optional flag obs_4_downward_P as the targeted pressure, for example:

Options(1)%obs_4_downward_P = Atmosphere(1)%level_pressure(n_Layers)

To output upwelling radiance at aircraft level, set optional flag Aircraft_Pressure as the pressure of aircraft, for example:

Options(1)%Aircraft_Pressure = Atmosphere(1)%level_pressure(80)

One can set either of these two flags, but not both at the same time. Without setting these optional flags, the output RTS%radiance will be radiance at the TOA level.

Note: to avoid confusion, another possibility is to set Aircraft_Pressure as obs_4_upward_P, or rename these flags. Something to discuss later.

[chengdang]

The existing test Test_CRTM_Aircraft require all standalone CRTM modules. Updated configuration files with PR #161.
Will first run this test to make sure the code updates are accurate.

[chengdang]

Updated aircraft unit and added downwelling radiance test with commit 8a9e36f

Note that these tests only proof that we can use these flags for CRTM calculation. More scientific tests are needed.