Enforce units in YAML conditions

We should consider having (and enforcing) explicit units in the YAML condition files. Not every constant has a dimension so it must be clear somehow where a unit is required.

It might be heavy to add e.g. *mm to every relevant number so some contractions might help. Some ideas can be:

VPLeft: !alignment
  position: [0.0, 0.0, 0.0]*mm
  rotation: [0.0, 0.0, 0.0]

units:
  position: mm
VPLeft: !alignment
  position: [0.0, 0.0, 0.0]
  rotation: [0.0, 0.0, 0.0]

The motivation is to get rid of implicit conversions such as https://gitlab.cern.ch/lhcb/Detector/blob/c488d1e2aae7db3fd9eb15fb82c4deb807d663d3/Core/src/yaml_converters.cpp#L44

/cc @sponce @clemenci