@type t() :: %ExtFit.Types.FieldData{
  field: nil | ExtFit.Types.Field.t(),
  field_def: nil | ExtFit.Types.FieldDefinition.t(),
  parent_field: nil | ExtFit.Types.Field.t() | ExtFit.Types.Subfield.t(),
  raw_value: nil | non_neg_integer(),
  units: nil | String.t(),
  value: any(),
  value_label: nil | String.t()
}