Rename component Core -> Sim

We are still calling everything below GUI the "core" of BornAgain. This conflicts with using "Core" in the name of one of the components that make up "core".

Hence my proposal to call the latter "kernel" instead. However, we failed to consequentially adopt this new name. Therefore, let's fall back to "core" vs "GUI", and rename the "Core" core component to something else. Proposal: Sim.