Fix for https://github.com/McStasMcXtrace/McCode/issues/1741

[willend]

This in effect means:

• Partial revert of f9c8442 and bc50576
• Documenting that tth_sign only works together with d_phi

I further found that weigh corrections were not correctly applied with tth_sign, powder lines became a factor of 2 too strong.

[farhi]

Thanks Peter, Would be nice to port as well to X-rays ???

[willend]

Yup, I will have a look at that also.

[willend]

(But may not happen until next week, so will simply keep the issue open)