Missing Sample implementation for custom types U11, U20, I11, I20 #166

The dasp crate does not provide an implementation of the trait Sample for the types U11, U20, I11, I20 defined in dasp::sample::types, despite the trait being implemented for the types U24, U48, I24, I48.

Not sure if this is an oversight or a deliberate omission, if it is an oversight it should be implemented.