Some docs for critical

doc/source/algorithms/critical_curves.ipynb

@@ -48,6 +48,19 @@
     "modelPR.solve_pure_critical(T0, rho0)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "9141858c",
+   "metadata": {},
+   "source": [
+    "If you have a mixture, but want to obtain the critical point of a pure fluid of this mixture, you can specify the index of the component in the mixture, as well as the number of componnts in the mixture with something like:\n",
+    "\n",
+    "``\n",
+    "model.solve_pure_critical(T0, rho0, {\"alternative_pure_index\": 1, \"alternative_length\": 2})\n",
+    "``\n",
+    "so here, for the second fluid, with 0-based index of 1, in a two-component mixture"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "e15eeded",