moves initialization assert to better location

tests/DNS/test_scaling.py

@@ -21,7 +21,6 @@ def get_DNS_parameters(
         src_iteration = None,
         kMeta = 1.5,
         fftw_plan_rigor = 'FFTW_MEASURE'):
-    assert (N in [128, 1024, 2048, 4096])
     simname = (DNS_type + '{0:0>4d}'.format(N))
     if output_on:
         simname = DNS_type + simname
@@ -36,6 +35,7 @@ def get_DNS_parameters(
         class_name += '_no_output'
     src_simname = src_prefix + '_N{0:0>4d}_kMeta{1:.1f}'.format(N, kMeta)
     if type(src_iteration) == type(None):
+        assert (N in [1024, 2048, 4096])
         if N == 1024:
             src_iteration = 32*1024
         if N == 2048: