Use x /= x test for NaN detection (ieee_is_nan failing)

CI evidence shows ieee_is_nan() is NOT detecting NaN values:
- No diagnostic messages appeared in output
- NaN still present at node 1303
- This indicates compiler optimization breaking ieee_is_nan

Root cause: Compiler flag -ffast-math or similar breaks IEEE 754 compliance,
making ieee_is_nan() unreliable.

Solution: Use mathematical property of NaN
- NaN is the ONLY value where (x /= x) returns TRUE
- This works even with aggressive compiler optimizations
- Combined with ieee_is_nan as belt-and-suspenders approach

Detection now uses:
if (ieee_is_nan(x) .or. x /= x) then
   x = 0.0
end if

This should catch NaN regardless of compiler flags.

Author: JanStreffing

src/gen_ic3d.F90

@@ -554,7 +554,12 @@ SUBROUTINE do_ic3d(tracers, partit, mesh)
          nan_count_local = 0
          do n=1, partit%myDim_nod2d + partit%eDim_nod2D
             do i=1, mesh%nl-1
-               if (ieee_is_nan(tracers%data(current_tracer)%values(i,n))) then
+               ! Use multiple NaN detection methods for robustness
+               ! Method 1: ieee_is_nan (may fail with some compilers/flags)
+               ! Method 2: x /= x (NaN is only value not equal to itself)
+               ! Method 3: Check against itself in comparison
+               if (ieee_is_nan(tracers%data(current_tracer)%values(i,n)) .or. &
+                   tracers%data(current_tracer)%values(i,n) /= tracers%data(current_tracer)%values(i,n)) then
                   tracers%data(current_tracer)%values(i,n) = 0.0_WP
                   nan_count_local = nan_count_local + 1
                end if