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

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