Comparison of three fit method for extracting D_TT

1, Fit D_TT with a constant.

2, Fit the D_TT*cos(theta*) with a linear function ( y = p0 + p1 * cos(theta* ) ).

3, Fit the D_TT*cos(theta*) with a linear function ( y = p0 * cos(theta*) ).

The three fitting results are consistent with each other very well.