MultipoleT Cleanup and Placement

Currently MultipoleT places from the middle. Would be nice if it were possible to place from the start, as with other elements. Also some cleanup would be useful (and improved docs).

assigned to @ext-rogers_c

created branch 794-multipolet-cleanup-and-placement to address this issue

mentioned in merge request !649

Just going through this. Jobs:

1. migrate to a more generalised fringe field model; at the moment the fringe field is hardcoded to some asymmetric tanh model. Martin Tat implemented a better tanh model for higher derivatives which should be brought into the generalised EndFieldModel tanh and should enable e.g. AsymmetricEnge etc
2. make sure that the interface is consistent with e.g. ScalingFFAMagnet. This might mean adding some extra UI variables to ScalingFFAMagnet.
3. BoundingBox concept is confused with placement. Placement is done from the middle. Need to add a parameter that gives an offset and do placement from the magnet entrance. In ScalingFFAMagnet this is MAGNET_START parameter (distance to the opening fringe field) and MAGNET_END parameter (distance from closing fringe field to the next element). "Offset" element then let's us do horizontal offsets and rotations. @carl_j pointed out that this is inconsistent with the Opal-T way (ELEMEDGE et al)...
4. HAPERT and VAPERT are inconsistent with OpalElement APERTURE and ScalingFFAMagnet
5. Time dependence would be nice for pulsed magnet systems (kickers, RCS, etc)
6. General testing. I would like to check that the magnet model makes sense and converges to Maxwell as the field expansion order increases.
7. GlobalCartesianOffset is only enabled for PyOpal, I should make sure that it is usable for non-python Opal-Cycl

I may find some more as I go (will edit this comment). I wonder how much this model is used, if I need to maintain backwards compatibility.

For info, the way ScalingFFAMagnet is placed is shown in this plot:

The point (R, phi) = (4.0, 0) is the position of the placement pointer just before the FFA magnet placement. The point (R, phi) = (4.0, magnet_end/R0) is the position of the placement pointer just after the FFA magnet placement. R0 is 4.0 m for this magnet and lengths are defined in metres (not degrees or radians)