STY: Capitalize the mask warning message variable name

Capitalize the mask warning message variable name for the sake of
consistency with the rest of the global variables across the code base.

Document its meaning.

Author: jhlegarreta

src/nifreeze/model/base.py

@@ -27,9 +27,10 @@
 
 import numpy as np
 
-mask_absence_warn_msg = (
+MASK_ABSENCE_WARN_MSG = (
     "No mask provided; consider using a mask to avoid issues in model optimization."
 )
+"""Mask warning message."""
 PREDICTED_MAP_ERROR_MSG = "This model requires the predicted map at initialization"
 """Oracle requirement error message."""
 UNSUPPORTED_MODEL_ERROR_MSG = "Unsupported model <{model}>."
@@ -104,7 +105,7 @@ def __init__(self, dataset, **kwargs):
         self._dataset = dataset
         # Warn if mask not present
         if dataset.brainmask is None:
-            warn(mask_absence_warn_msg, stacklevel=2)
+            warn(MASK_ABSENCE_WARN_MSG, stacklevel=2)
 
     @abstractmethod
     def fit_predict(self, index: int | None = None, **kwargs) -> np.ndarray | None:

test/test_model.py

@@ -40,9 +40,9 @@
 )
 from nifreeze.model._dipy import GaussianProcessModel
 from nifreeze.model.base import (
+    MASK_ABSENCE_WARN_MSG,
     PREDICTED_MAP_ERROR_MSG,
     UNSUPPORTED_MODEL_ERROR_MSG,
-    mask_absence_warn_msg,
 )
 from nifreeze.testing import simulations as _sim
 
@@ -98,7 +98,7 @@ def test_trivial_model(request, use_mask):
         mask = np.ones(size, dtype=bool)
         context = contextlib.nullcontext()
     else:
-        context = pytest.warns(UserWarning, match=mask_absence_warn_msg)
+        context = pytest.warns(UserWarning, match=MASK_ABSENCE_WARN_MSG)
 
     _S0 = rng.normal(size=size)