Dipoles in Opal-T to support quadrupole component field gradients

The following discussion from !269 (merged) should be addressed:

• @snuverink_j started a discussion: (+3 comments)

  The commenting out was introduced by @kraus in 595b4b83 .. perhaps he can comment. Note that the class at that time was called Bend : https://gitlab.psi.ch/OPAL/src/blob/595b4b83818596b5f7a72e086cbbda4325f70aa8/src/Classic/AbsBeamline/Bend.cpp

• @kraus The dipoles in Opal-T used to support field gradients in version 1.6 and before. When I rewrote Opal-T I disabled it to speed up the development. However it was my intention to re-enable it (but didn't happen). I still think that we should support the quadruple component.