restore Quantity conversion

need to wait for next monty release

src/pyEQL/solution.py

@@ -2309,6 +2309,10 @@ def as_dict(self) -> dict:
         if self.volume_update_required:
             self._update_volume()
         d = super().as_dict()
+        for k, v in d.items():
+            # convert all Quantity to str
+            if isinstance(v, Quantity):
+                d[k] = str(v)
         # replace solutes with the current composition
         d["solutes"] = {k: f"{v} mol" for k, v in self.components.items()}
         # replace the engine with the associated str