Add a 3-phase formula type that wraps 3 1-phase formulas

This also adds an evaluator and tests for the 3-phase formulas.

Signed-off-by: Sahas Subramanian <[email protected]>

Author: shsms

src/frequenz/sdk/timeseries/formulas/__init__.py

@@ -2,3 +2,8 @@
 # Copyright © 2025 Frequenz Energy-as-a-Service GmbH
 
 """Formulas on telemetry streams."""
+
+from ._formula import Formula
+from ._formula_3_phase import Formula3Phase
+
+__all__ = ["Formula", "Formula3Phase"]