use less particles in test

tox_plain.ini

@@ -12,7 +12,7 @@ commands =
     #python tests/test_plain.py -n 256 --run --initialize --ncpu 8 --niter_todo 8 --precision single --wd "data/single"
     python tests/test_plain.py \
         -n 32 --run --initialize --ncpu 4 \
-        --nparticles 10000 --niter_todo 64 \
+        --nparticles 100 --niter_todo 64 \
         --precision single --wd "data/single"
     #python tests/test_plain.py -n 32 --run --initialize --multiplejob --ncpu 2 --nparticles 16 --niter_todo 64 --precision single --wd "data/single"
     #python tests/test_plain.py -n 32 --run --initialize --multiplejob --ncpu 2 --nparticles 16 --niter_todo 64 --precision double --wd "data/double"