subtable keywords are broken

[mweastwood]

When I switched from wrapping TableProxy to just regular Table, I missed a conversion that TableProxy was doing automatically. I mistakenly assumed Table would do the same thing.

When reading a keyword, TableProxy does the following if the keyword points to another table.

  ...
  case TpTable:
    return ValueHolder ("Table: "+keySet.tableAttributes(fieldId).name());
  ...

Because I'm now wrapping Table, I could probably just return the actual Table object itself, which seems like the sensible thing to do. However at the moment it is just broken because I haven't defined a wrapper function for the case where the data type is TpTable.