McStas 2.x + 3.x: Field-precession with gravity is not supported

[willend]

Currently, the combination of propagation including both a precession-field and gravitation is not supported. (This means that e.g. Guide_gravity can not be placed in a field-region.)

In 2.7.1 the user will get a vague warning pr. neutron, in 3.1 the issue is silently ignored.

As a minimum this limitation should be documented properly.