Vectorization error in RBend

[Hespe]

On the current master branch (version 0.7, commit b9f971d) the RBend segment fails to be properly constructed.
Likely related to #170.

Minimal example:

import torch
import cheetah

segment = cheetah.Segment(
    elements=[
        cheetah.RBend(length=torch.tensor([1])),
    ]
)

segment.plot_overview()

Crashes with IndexError: too many indices for tensor of dimension 0