Split and merge output

[felixhekhorn]

Continuation of #96

We want

• merge separately computed (but input compatible) outputs
• split a single output into multiple ones (not strictly needed, but it's dual to the former one, and so nice to have)

This can be especially relevant for "the big eko" which contains all datasets

[alecandido]

I marked as post milestone because it would a non-breaking addition to the public interface. If done before, so much the better.