dem_error: set cond=1e-8 to fix abnormal fit residual + add unit test

[yunjunz]

Description of proposed changes

• dem_error: fix abnormal residual TS with default phase velocity mode

  • estimate_dem_error(): switch cutoff value of the least squares solution from 1e-15 to 1e-8, to avoid abnormal high TS residual value, when the phase velocity mode is turned ON by default (dem_error: change default from phase to phase velocity #875). Tests (based on the new unit test) shows this change of cutoff value has no impact on the estimation result accuracy at all, which is in contrast to the brief conclusion in e28c240.
  • set debug_mode as global variable to be test a specific pixel given Y/X easily.

• add tests/dem_error.py, to test the DEM error estimation / correction in both scenarios (simple linear and complex non-linear) for one pixel. functions/modules for unit test #402

  • add to circle CI testing

• tropo_pyaps3: improved auto-skip/run checks

  • skip dload if ERA5.h5 already exists
  • skip re-applying correction if corrected displacement exists and is newer
  • print out msg adjustment

Reminders

• Pass Codacy code review (green)
• Pass Pre-commit check (green)
• Pass Circle CI test (green)
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• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.