timing.py sign error

[SixByNine]

Investigations within the EPTA (Siyuan, Aurelien, Stas and myself) suggest that the sign of the return from tm_delay() in timing.py is incorrect.

This is because a signal_base.function should return the "positive" function that is being fit, rather than a correction to be applied to the residuals. This means that the corrected residual should be given by:

newres = residuals - retval

which implies that

retval = residuals - newres

and so return new_res - residuals on line 47 should be return residuals - new_res.

This has been verified by testing against pulsars with known parameter offsets.

[AaronDJohnson]

Has this been resolved @SixByNine ?