Resolve "CPU only runs on GPU machines should fail by default"

Description

Runs on a GPU machine are now failing in case that the GPU gets disabled by some conditions, such as curvilinear meshes, or other features not yet implemented for the GPU. A new option (AllowCPUonly) is introduced, which allows to override this behaviour.

News snippet

Runs without using the GPU will now by default fail on GPU machines.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Closes #251 (closed)