Move the xc code to its own folder and fix few small things

Description

This merge request introduces the following thing:

• Move the xc code to its own folder
• The size inconsistent functionals are now detected by the code
• Forbid to run size-inconsistent functional with periodic systems.
• Forbid to run Yukawa hybrids, as they are not supported
• Fix a bug with CAM hybrids with external parameters for spin polarized electrons
• Adding an explicit error message for SIC when not KS-DFT.
• Forbid SIC with size inconsistent functionals.
• Forbid to compute the stress for potential-only xc functionals

Some cleaning is also done.

Closes #981 (closed)

News snippet

Improve the XC code and fix few small bugs

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.