Draft: [Fix] Read velocity from field in UBB

In the UBB boundary condition, the velocity is taken from neighbouring points when a field is provided. However, this also results in prescribing a different velocity for PDFs of the same cell which can not be intentional. With this MR the velocity is taken from the source cell instead

requested review from @ed94aqyc

assigned to @holzer

added 1 commit

• 93e8eb99 - simplify condition

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resolved all threads

approved this merge request

marked this merge request as draft

cupy in pipeline was fixed with !185 (merged), so you can merge from master to finish this one and !184