Incorrect dtshift in Exact with shift_add_{x,y,z} parameters

The Exact thorn has shift_add_{x,y,z} parameters for adding a constant to the shift. Considering these as a coordinate transformation using a vector v^i = [shift_add_x, shift_add_y, shift_add_z], such a transformation should affect both the shift and its time derivative. This is because the time derivative in the new coordinates differs from the time derivative in the old coordinates by a term v^j d(beta^i)/dx^j. This is correctly implemented as a 4D coordinate transformation in the EinsteinExact thorns and I have verified that the difference relative to the Exact thorn is given by this missing term.

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