RBend requires beampipe thickness

[Laurie Nevay]

If the beampipeThickness option is not set, and the lattice contains an RBend, BDSIM will exit on a G4Exception caused by the beampipe torus having the same outer and inner radius. This behaviour does not match any of the other elements.

RBend builds the geometry of the beampipe itself, whereas all other elements inherit the BuildBeampipe method from BDSMultipole.

[Jochem Snuverink]

the default beampipethickness is 0. Should we change this to a default value, e.g. the minimum value necessary. Or would it be better to print a warning and exit properly when the beampipethickness is not set?

Btw, also Sectorbend has a special BuildBeampipe method. It would be better if the buildbeampipe method would have a common virtual interface and can be overwritten by each element.

[Jochem Snuverink]

the tolerance was set on the inner instead of the outer radius. It is solved now in the latest push.