ENH: implemented support for column size.
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