NB: 3 cases may occur: size unused, allowed or mandatory. In the
     latter case, if no size is provided, then a default value (#-defined)
     is assigned.
ENH: added minimal support for table preambles.
ENH: moved the GetColumnType methods to the internals of ExecuteSchema.
     This results in a smaller API, at the expense of a little flexibility.
ENH: refined support for various types at the concrete class level