migrate CoreMetadata#physical_medium #format

[rococodogs]

to migrate values, run the following from a rails console:

ActiveFedora::Base.where(physical_medium: '[* TO *]').each do |work|
  work.format = work.format + work.physical_medium
  # work.physical_medium = []
end

note: before removing the property from our model mixin, be sure to clear out physical_medium values using work.physical_medium = []; this prevents the values from being persisted to fedora where they'd be unreachable