Added a few metrics for measuring performance

spatz/metrics/mse.py

@@ -0,0 +1,15 @@
+from numpy.typing import ArrayLike
+from spatz.metrics import Metric
+
+
+class MSEMetric(Metric):
+    def __init__(self) -> None:
+        """Mean squared error
+        """
+        super().__init__()
+
+    def _update(self, x: ArrayLike, y: ArrayLike):
+        if self._score is None:
+            self._score = 0.5 * (x - y)**2
+        else:
+            self._score += 0.5 * (x - y)**2