assert helping to investigate R&S '82 example failures on Windows (#1541)

Author: slayoo

examples/PySDM_examples/Rozanski_and_Sonntag_1982/figs_4_5_6.ipynb

@@ -173,6 +173,7 @@
     "                d_n_vapour=-self.delta_liquid_water_mixing_ratio / self.dt,\n",
     "                n_liquid=FIG4_CAPTION_PARAMS['autoconversion_mixrat_threshold']  # TODO #1207\n",
     "            )\n",
+    "            assert dRv__dt[isotope] <= 0\n",
     "            \n",
     "        super().advance_parcel_vars()\n",
     "\n",