[MONDRIAN-1816] Specifying a <ColumnDef> would cause a "duplicate column" error, preventing the ability to specify type and/or internalType for physical columns.

This was caused by a check in RolapSchemaLoader.registerColumns, which checked whether the columnsByName collection already contains the column.  That's a valid check
for calculated columns or columns on an inline table, but was preventing execution from ever hitting the final else block of this method, which sets the type of
existing physical columns.
Merge pull request #196 from mkambol/lagColumnDef

[MONDRIAN-1816] Specifying a <ColumnDef> would cause a "duplicate column...