USAGE : check_datatypes  myfile.tdt [my_datatypes_database]

        If my_datatypes_database is given it must be fully defined as
        The default is to update VCS_datatypes@vcs

  The input myfile.tdt will usually have come from conversion of an MDL
  SD file using build_trees().

  The data tags are extracted and checked for existence in the datatypes
  database. If they do not exist the the user is asked to supply the
  required information to construct the tags. When these have all been
  collected they are loaded into the database.

  No check is made to ensure that the data tags are fully equivalent.