`quantity_spec` members

[mpusz]

quantity_spec is the only SymbolicConstant that has public members (dimension and character). named_unit, point origins, other SymbolicConstants, and even reference do not expose anything. Only quantity and quantity_point expose public members.

Should we continue doing that, or should we provide an alternative interface to obtain this information (e.g., get_dimension(QuantitySpec), get_character(QuantitySpec))?