Add back Offset property for DumpBeamline

Dorothea Vom Bruchrequested to merge
dovombru_reenable_DumpBeamline_offset into master

The property to manually shift the beamline within the DumpBeamline algorithm was removed with 3de89a93.

While the position of the beamline should always be described correctly by the conditions tag used to process a sample, the property can be useful for misalignment studies and quick checks.

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