fix: broken source imports (#450) (#454)

## Description
It looks like a couple of source imports were broken during #430, this
fixes the import errors.

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Author: aaron-hopkinson

src/anemoi/datasets/create/sources/accumulations.py

@@ -998,12 +998,12 @@ def _execute(
             and request.get("stream", "oper") == "oper"
             and request.get("accumulation_period") == 24
         ):
-            from .accumulations2 import accumulations as accumulations2
+            from .accumulations2 import Accumulations2Source
 
             LOG.warning(
                 "🧪️ Experimental features: Using accumulations2, because class=ea stream=oper and accumulation_period=24"
             )
-            return accumulations2(context, dates, **request)
+            return Accumulations2Source._execute(context, dates, **request)
 
         _to_list(request["param"])
         class_ = request.get("class", "od")

src/anemoi/datasets/create/sources/tendencies.py

@@ -110,8 +110,7 @@ def _execute(dates: list[datetime.datetime], time_increment: Any, **kwargs: Any)
         shifted_dates = [d - time_increment for d in dates]
         all_dates = sorted(list(set(dates + shifted_dates)))
 
-        # from .mars import execute as mars
-        from anemoi.datasets.create.mars import execute as mars
+        from .mars import mars
 
         ds = mars(dates=all_dates, **kwargs)