Flatten the directory structure

[helen-brooks]

Context

The nested structure of aurora and aurora/openmc is an historical artifact of development, and doesn't really serve any purpose. The conceptual separation of performance runs can still be achieve via input file syntax. We should flatten the directory structure.

Motivation

• It's a bit confusing as-is
• Will avoid the need to link (both at compile and run-time) two different applications
• Will facilitate further MOOSE-app couplings
• Will make unit testing set-up more sensible