Mesh generator that checks/adds sidesets around a set of blocks

[snschune]

Reason

I need the ability to do two things:

1. Check if a set of blocks is enclosed by a set of sidesets.
2. Add faces that are not covered by one of the provided sidesets to a new sideset.

This is useful for radiative heat transfer in the heat conduction module and potentially for
fluid flow problems defined on portions of the domain.

Design

Mesh generator that takes a list of blocks and boundaris. Optionally a new sideset name
can be provided, if not provided, an error is thrown if an outside bpundary of the set of blocks
is not covered by one of the sidesets.

Impact

new capability.