Fix liquidity_time_bucket attribute on Universe (#194)

- Needs to be an alias

tradingstrategy/universe.py

@@ -65,9 +65,6 @@ class to pass around information of its available data.
     #: Historical candles for the decision making
     candles: Optional[GroupedCandleUniverse] = None
 
-    #: Liquidity data granularity
-    liquidity_time_bucket: Optional[TimeBucket] = None
-
     #: Historical liquidity samples.
     #:
     #: Might not be loaded if the strategy does not need to access
@@ -137,6 +134,14 @@ def __post_init__(self):
         for c in self.chains:
             assert isinstance(c, ChainId), f"Expected ChainId object, got {c.__class__}: {c}"
 
+    @property
+    def liquidity_time_bucket(self) -> TimeBucket:
+        """Alias for liquidity.time_bucket.
+
+        Legacy code compat.
+        """
+        return self.liquidity.time_bucket
+
     @property
     def lending_reserves(self) -> LendingReserveUniverse | None:
         """Each lending metric is paired with a copy of the universe