fix problem when using spinors and independent particles
Description
When using spinors and independent particles, there was a seg fault in the output of xc_torque. This MR forbids the combination of xc_torque output theory levels, which do not define vxc.
News snippet
Fix problem with xc_torque output for independent particles.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.