Add support for Velo Motion System override from YAML in PyConf

This is a follow up from !3673 (merged).

Here I

• fix a minor limitation in !3673 (merged) (the support of units in alignment conditions was missing)
  • note this is a small correction only for consistency as at this time units are not used in the Online generated conditions: https://gitlab.cern.ch/lhcb-conddb/lhcb-conditions-database/-/blob/master/Conditions/VP/Motion.yml/240052
• fix the logic for IOVLock generation (0578c1db)
  • see discussion at https://mattermost.web.cern.ch/lhcb/pl/d3a9de4zriynmygjdsdcyghjza
• add support for Velo Motion System override from YAML in PyConf
  • only in DetDesc based builds and conditional to options.velo_motion_system_yaml

/cc @sborghi