Improve ConverterNamespace

* [ ] If used as a `Function`, all `Converter`s in the `ConvertNamespace`use `Map` to create the output. However, we should use `MapView' whenever we can (lazy).

* [ ] If used as a `Computer` the expected output has to be an `Img`. However, it just needs to be `IterableInterval`.

* [ ] Provide methods for customized conversion on `IterableInterval` or `RAI` level (e.g. more than just copy the values over), e.g. scale input values to destination type, normalize values and scale values to destination type, etc.

Start by reading ConverterNamespace and the related ConvertNamespace Function and Computer entry points. Clarify and implement the three proposed areas—lazy MapView use, IterableInterval output support, and customized conversion operations—then verify each requirement with the project’s existing tests.

Written by the indexing model from the issue text.