Combiner generator should not always merge sidesets and boundaries

[GiudGiud]

Motivation

Carolina was trying to have several parts of a circle be meshed independently then combined while preserving boundaries.

But the combiner merges everything with the same ids.

Design

So either:

• force users to see it merged, be unhappy, then use a RenameBoundary/BlockGenerator then it works
• add offset id/names features to EVERY SINGLE mesh generator (you do that not me)
• add simple offsets to the merge generator that combine (combiner first, maybe stitcher soon)

Impact

Easier mesh generation workflows

[GiudGiud]

Actually I might do option 2 for the subdomains of a GMG. I kind of could use it for making a bunch of cylinders. But in general I think not-merging is the way to go, less efforts for the same result