smallbaselineApp/dem_error: roll back to "phase" minimization as default

[yunjunz]

Description of proposed changes

• mintpy/defaults/smallbaselineApp(_auto).cfg: roll back the default to phase velocity for topo resid (dem_error: change default from phase to phase velocity #875), as discussed in abnormal large residual from dem_error for timeseries_rms #892.

• tests/dem_error: use model with step functions, which has better est. accuracy for complex deformation

• mintpy/utils/time_func/design_matrix4poly: more comments on the math form of the polynomial equations

Reminders

• Pass Codacy code review (green)
• Pass Pre-commit check (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.