Remove C_SINGLE_ATTRIBUTE and C_MULTIPLE_ATTRIBUTE classes

The classes C_SINGLE_ATTRIBUTE and C_MULTIPLE_ATTRIBUTE add complexity, but not much value, because both allow multiple child C_OBJECTs, since even single-valued attributes have multiples, they are just treated as possible alternatives for one item rather than for more than one.

It would therefore make sense to fold them back into the class C_ATTRIBUTE.