Fix a bug in the offset of the spiral phase on GPUs.

Description

Fix a bug in the offset of the spiral phase on GPUs. The energy after the magnon kick was very different and is now much closer to the one before the kick, as it should be. It also matches the CPU energy.

News snippet

Fix a bug for the magnon kick on GPUs.

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.

testsuite/functionals/18-mgga.test

@@ -32,8 +32,8 @@ if (available libxc5); then
   match ;  External energy    (libxc5) ; GREPFIELD(static/info, 'External    =', 3) ; -20.05794552000
   Precision: 7.50e-08
   match ;  Eigenvalue     [1] (libxc5) ; GREPFIELD(static/info, '1   --', 3) ; -1.4990320
-  Precision: 1.09e-13
-  match ;  energy_density  ; LINEFIELD(static/energy_density.y\=0\,z\=0, 11, 2) ; 13.1053043223782
+  Precision: 1.0e-10
+  match ;  energy_density  ; LINEFIELD(static/energy_density.y\=0\,z\=0, 11, 2) ; 13.1053043223538
 else
   Precision: 3.38e-07
   match ;  Total energy       (libxc4) ; GREPFIELD(static/info, 'Total       =', 3) ; -6.750548090000001
@@ -51,8 +51,8 @@ else
   match ;  External energy    (libxc4) ; GREPFIELD(static/info, 'External    =', 3) ; -20.058501030000002
   Precision: 7.50e-08
   match ;  Eigenvalue     [1] (libxc4) ; GREPFIELD(static/info, '1   --', 3) ; -1.499313
-  Precision: 1.09e-13
-  match ;  energy_density  ; LINEFIELD(static/energy_density.y\=0\,z\=0, 11, 2) ; 13.106048616038802
+  Precision: 1.0e-10
+  match ;  energy_density  ; LINEFIELD(static/energy_density.y\=0\,z\=0, 11, 2) ; 13.1060323566996
 endif