Forbid instantiating Quantity

We don't want Quantity to be instantiated neither by using `zero()` or
`from_string()`.

Signed-off-by: Leandro Lucarella <luca-frequenz@llucax.com>

Author: llucax

src/frequenz/sdk/timeseries/_quantities.py

@@ -97,7 +97,14 @@ def zero(cls) -> Self:
 
         Returns:
             A quantity with value 0.0.
+
+        Raises:
+            TypeError: If called on the base
+                [`Quantity`][frequenz.sdk.timeseries.Quantity] class.
         """
+        if cls is Quantity:
+            raise TypeError("Cannot instantiate a base Quantity class.")
+
         _zero = cls._zero_cache.get(cls, None)
         if _zero is None:
             _zero = cls.__new__(cls)
@@ -118,8 +125,12 @@ def from_string(cls, string: str) -> Self:
 
         Raises:
             ValueError: If the string does not match the expected format.
-
+            TypeError: If called on the base
+                [`Quantity`][frequenz.sdk.timeseries.Quantity] class.
         """
+        if cls is Quantity:
+            raise TypeError("Cannot instantiate a base Quantity class.")
+
         split_string = string.split(" ")
 
         if len(split_string) != 2: