implement general purpose mask filter (#20)

* add apply_mask filter

src/anemoi/transform/filters/apply_mask.py

@@ -0,0 +1,60 @@
+# (C) Copyright 2024 Anemoi contributors.
+#
+# This software is licensed under the terms of the Apache Licence Version 2.0
+# which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# In applying this licence, ECMWF does not waive the privileges and immunities
+# granted to it by virtue of its status as an intergovernmental organisation
+# nor does it submit to any jurisdiction.
+
+import earthkit.data as ekd
+import numpy as np
+
+from ..fields import new_field_from_numpy
+from ..fields import new_fieldlist_from_list
+from ..filter import Filter
+from . import filter_registry
+
+
+@filter_registry.register("apply_mask")
+class MaskVariable(Filter):
+    """A filter to mask variables using external file."""
+
+    def __init__(
+        self,
+        *,
+        path,
+        mask_value=1,
+        threshold=None,
+        rename=None,
+    ):
+
+        mask = ekd.from_source("file", path)[0].to_numpy().astype(bool)
+
+        if threshold is not None:
+            self._mask = mask > threshold
+        else:
+            self._mask = mask == mask_value
+
+        self._rename = rename
+
+    def forward(self, data):
+
+        result = []
+        extra = {}
+        for field in data:
+
+            values = field.to_numpy(flatten=True)
+            values[self._mask] = np.nan
+
+            if self._rename is not None:
+                param = field.metadata("param")
+                name = f"{param}_{self._rename}"
+                extra["param"] = name
+
+            result.append(new_field_from_numpy(values, template=field, **extra))
+
+        return new_fieldlist_from_list(result)
+
+    def backward(self, data):
+        raise NotImplementedError("`apply_mask` is not reversible")