fix unit test (rank ==)

Author: spossann

src/struphy/pic/tests/test_sph.py

@@ -1039,7 +1039,7 @@ def du_xyz(x, y, z):
     err_uy = xp.max(xp.abs(all_velo2 - v2_e)) / xp.max(xp.abs(v2_e))
     err_uz = xp.max(xp.abs(all_velo3 - v3_e)) / xp.max(xp.abs(v3_e))
 
-    if comm.Get_rank() == 0:
+    if rank == 0:
         print(f"\n{boxes_per_dim = }")
         print(f"{kernel = }, {derivative = }")
         print(f"{bc_x = }, {eval_pts = }, {tesselation = }")