Use __slots__ in model classes to reduce memory consumption (and
detect attribute mistakes).
Added hxl.model.column.columnNumber and
hxl.model.column.sourceColumnNumber.

Next: eliminate hxl.model.value.columnNumber and hxl.model.value.sourceColumnNumber.
Eliminated hxl.model.HXLValue.columnNumber and
hxl.model.HXLValue.sourceColumnNumber (use values in hxl.model.HXLColumn).