Polarisation in McStas-3.x: No support for tabulated fields with embedded optics

[willend]

McStas-3.x limitation wrt. polarisation:

Currently, the Pol_Bfield component in McStas-3.x only supports "analytically defined" fields. The component may be "open-ended" by setting concentric=1, leaving field-exit to Pol_Bfield_stop - thus allowing to put e.g. optical components within "the sandwich".

Tabulated fields are currently only a feature of Pol_tabled_field which on the other hand does not extend beyond that "box".