Set get_activation by name to pure

Author: jvdp1

src/nf/nf_activation.f90

@@ -733,7 +733,7 @@ pure function eval_3d_celu_prime(self, x) result(res)
   end function eval_3d_celu_prime
 
   ! Utility Functions
-  function get_activation_by_name(activation_name) result(res)
+  pure function get_activation_by_name(activation_name) result(res)
     character(len=*), intent(in) :: activation_name
     class(activation_function), allocatable :: res
 
@@ -815,4 +815,4 @@ pure function get_name(self) result(name)
     end select
   end function get_name
 
-end module nf_activation
+end module nf_activation