dem_error w/ phase velocity: rm all-zero column in design matrix

[yunjunz]

Description of proposed changes

• dem_error.py --phase-velocity: remove the all-zero column in the design matrix G and the all-one column in the design matrix G0, as suggested by @mirzaees in abnormal large residual from dem_error for timeseries_rms #892. This is because the constant intercept term of the polynomial is not estimated in the phase velocity minimization scenario. Including this all-zero column in G could sometimes introduce abnormally large eigenvalues in the least-squares inversion, resulting in an unrealistic TS residual.

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.

[mirzaees]

Thank you @yunjunz! It looks good.
The only point that we should not forget is that timeseries residuals in line 266 lacks a constant if using phase velocity and results large residuals, hence not reliable.
It will be resolved after moving this part to velocity estimation