Add a simple way to check whether a backend is available

[david-zwicker]

This is particularly important to test for optional backends, like torch. This would probably aid user code eventually, but it would be particularly useful already in some tests, that quickly want to test compiled code.

[david-zwicker]

This should then be used in PDEBase.determine_backend to check whether a backend is available. A possible implementation might be added to the register_package method of the backend registry.

[david-zwicker]

If this is implemented, we could also introduce a global available_backends method, which lists all available backends.