Fix division-by-zero when computing statistics for a single-time data…

…set (#142)

When calculating sample indices, make sure that at least one sample
is included.

src/anemoi/datasets/data/dataset.py

@@ -469,7 +469,7 @@ def _compute_constant_fields_from_a_few_samples(self):
         sample_count = min(4, len(indices))
         count = len(indices)
 
-        p = slice(0, count, count // (sample_count - 1))
+        p = slice(0, count, count // max(1, sample_count - 1))
         samples = list(range(*p.indices(count)))
 
         samples.append(count - 1)  # Add last