Correct unit delay

[HansOlsson]

Closes #4451

[HansOlsson]

This is an alternative to #4650

Only merge one:

• If we want full backwards compatibility: Document how UnitDelay currently works. #4650 (and possibly add extra option)
• If we want something that works intuitively - this one.

[HansOlsson]

Note:
The reference file will need a new reference for unitDelay.y in ModelicaTest.Blocks.Discrete.

However, no need to have a reference for unitDelay1.y as it is asserted that is the same result as unitDelay.y; i.e,, the new UnitDelay-block works the same with and without a preceding Sampler (and with a preceding Sampler it works the same as before).