remove allocation in fftshift

Author: cval26

src/fftpack_fftshift.f90

@@ -5,7 +5,7 @@
     !> Shifts zero-frequency component to center of spectrum for `complex` type.
     pure module function fftshift_crk(x) result(result)
         complex(kind=rk), intent(in) :: x(:)
-        complex(kind=rk), allocatable :: result(:)
+        complex(kind=rk), dimension(size(x)) :: result
 
         result = cshift(x, shift=-floor(0.5_rk*size(x)))
 
@@ -14,7 +14,7 @@ end function fftshift_crk
     !> Shifts zero-frequency component to center of spectrum for `real` type.
     pure module function fftshift_rrk(x) result(result)
         real(kind=rk), intent(in) :: x(:)
-        real(kind=rk), allocatable :: result(:)
+        real(kind=rk), dimension(size(x)) :: result
 
         result = cshift(x, shift=-floor(0.5_rk*size(x)))