* Reworking where sigma_sqd_w and gamma_Skw_fnc are computed - this should all be BFB. Also fixing the G_unit test that was failing due to sigma_sqd_w issues
* Correcting arg list
* Fixes after rebase
* Making sigma_sqd_w_zt BFB vs master when using iiPDF_type=7 (new hybrid)
* Fixing python API
* Changing name of compute_xp3 to diagnose_xp3 and moving a1_coef and a3_coef calculation back into advance routines