Add 3-phase current formulas (#164)

This PR introduces some new types for streaming 3-phase data:
- `Sample3Phase` :: for holding a 3-phase measurement.
- `FormulaChannel3Phase/FormulaReceiver3Phase` :: composable
channel/receiver types for streaming `Sample3Phase` objects.
- `FormulaEngine3Phase` :: for combining `Sample` objects from
`FormulaEngine` into `Sample3Phase` objects.
- `HigherOrderFormulaBuilder3Phase` :: For composing
`FormulaReceiver3Phase` and generating new `FormulaEngine3Phase` from
them.

It also adds formula generators for 3-phase `grid_current` and
`ev_charger_current`, and exposes them through the `LogicalMeter`.

Author: sahas-subramanian-frequenz

src/frequenz/sdk/timeseries/__init__.py

@@ -8,8 +8,6 @@
 [`Sample`][frequenz.sdk.timeseries.Sample]s.
 """
 
-from ._base_types import Sample
+from ._base_types import Sample, Sample3Phase
 
-__all__ = [
-    "Sample",
-]
+__all__ = ["Sample", "Sample3Phase"]

src/frequenz/sdk/timeseries/_base_types.py

@@ -26,3 +26,25 @@ class Sample:
 
     value: Optional[float] = field(compare=False, default=None)
     """The value of this sample."""
+
+
+@dataclass(frozen=True)
+class Sample3Phase:
+    """A 3-phase measurement made at a particular point in time.
+
+    Each of the `value` fields could be `None` if a component is malfunctioning
+    or data is lacking for another reason, but a sample still needs to be sent
+    to have a coherent view on a group of component metrics for a particular
+    timestamp.
+    """
+
+    timestamp: datetime
+    """The time when this sample was generated."""
+    value_p1: Optional[float]
+    """The value of the 1st phase in this sample."""
+
+    value_p2: Optional[float]
+    """The value of the 2nd phase in this sample."""
+
+    value_p3: Optional[float]
+    """The value of the 3rd phase in this sample."""