implement DeepHoldIn node

[falkhofmann]

Another idea is some more merge nodes.

As a start a DeepHoldIn would be handy. Basically very similar to the DeepMerge set to Holdout. but rather than a "stencil" it would be a "mask" in Merge terms.

If we get a proper set up for combining multiple deeps with each other, i might have some more ideas.
so far i am not aware of a good approach of even mimic a deep merge or holdout.

[charlesangus]

The idea being the Hold in would be the samples which aren't in the HoldOut, basically? Yeah, I've definitely wanted that before.