Make it easy for users to disable a shape in a Klee input file

[kennyweiss]

When we apply replacement rules in Klee-based shaping, we can add a list of materials that the current shape replaces or does_not_replace.

It can be convenient to allow users to "disable" a shape by indicating that it does not replace anything. E.g. this can help with debugging.

One way to do this might be to supply an empty list of materials that the current shape replaces. E.g.

shapes:
- name: some_shape
  material: some_material
  replaces: []  # suggestion: an empty list implies that this shape does not replace anything

This might be error prone (e.g. if a user intends to add materials the replaces list but forgets), so I'm open to other suggestions.

Note: The code currently ignores an empty list. See:

• https://github.com/LLNL/axom/blob/80889b6ca2248ad8abcaedca19f87593a4498320/src/axom/klee/Shape.cpp#L56-L67

To clarify: I'm suggesting a semantic difference between not supplying a replaces list (in which case, the default is that it replaces everything that's currently shaped in), and supplying an empty replaces list (which would imply that it replaces nothing).