Add support for adding non-interleaved samples

[sdroege]

See also https://github.com/sdroege/ebur128/issues/5

Main question here is how this would ideally look like API-wise.

[sdroege]

Options are:

• Single slice plus a channel stride
• A slice of slices, one per channel

[sdroege]

Found a way to add this relatively easily without code duplication.