fix: ensure missing value indicator in not a valid value in accumulat… (#316)

…ions

## Description

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## Type of Change

-   [x] Bug fix (non-breaking change which fixes an issue)
-   [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
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-   [ ] Documentation update

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## Code Compatibility

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### Code Performance and Testing

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- [ ] I ran the [complete Pytest
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suite locally, and they pass

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Co-authored-by: Aaron Hopkinson <aaron.hopkinson@ecmwf.int>

Author: b8raoult

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

@@ -26,6 +26,7 @@
 from .mars import mars
 
 LOG = logging.getLogger(__name__)
+MISSING_VALUE = 1e-38
 
 
 def _member(field: Any) -> int:
@@ -168,6 +169,7 @@ def write(self, template: Any) -> None:
         # are used to store the end step
 
         edition = template.metadata("edition")
+        assert np.all(self.values != MISSING_VALUE)
 
         if edition == 1 and self.endStep > 254:
             self.out.write(
@@ -176,6 +178,7 @@ def write(self, template: Any) -> None:
                 stepType="instant",
                 step=self.endStep,
                 check_nans=True,
+                missing_value=MISSING_VALUE,
             )
         else:
             self.out.write(
@@ -185,6 +188,7 @@ def write(self, template: Any) -> None:
                 startStep=self.startStep,
                 endStep=self.endStep,
                 check_nans=True,
+                missing_value=MISSING_VALUE,
             )
         self.values = None
         self.done = True
@@ -205,9 +209,6 @@ def add(self, field: Any, values: NDArray[Any]) -> None:
         if step not in self.steps:
             return
 
-        if not np.all(values >= 0):
-            warnings.warn(f"Negative values for {field}: {np.nanmin(values)} {np.nanmax(values)}")
-
         assert not self.done, (self.key, step)
         assert step not in self.seen, (self.key, step)