Nested config, devices registry

* All config now nested.
    - Use default config from model field default values. If providers
      should be enabled by default, non-empty default config file could
      be provided again.
    - Environment variable support with EOS_ prefix and __ between levels,
      e.g. EOS_SERVER__EOS_SERVER_PORT=8503 where all values are case
      insensitive.
      For more information see:
      https://docs.pydantic.dev/latest/concepts/pydantic_settings/#parsing-environment-variable-values
    - Use devices as registry for configured devices. DeviceBase as base
      class with for now just initializion support (in the future expand
      to operations during optimization).
    - Strip down ConfigEOS to the only configuration instance. Reload
      from file or reset to defaults is possible.

 * Fix multi-initialization of derived SingletonMixin classes.

src/akkudoktoreos/prediction/interpolator.py

@@ -6,6 +6,8 @@ from pathlib import Path
 import numpy as np
 from scipy.interpolate import RegularGridInterpolator
 
+from akkudoktoreos.core.coreabc import SingletonMixin
+
 
 class SelfConsumptionProbabilityInterpolator:
     def __init__(self, filepath: str | Path):
@@ -67,5 +69,17 @@ class SelfConsumptionProbabilityInterpolator:
     #     return self_consumption_rate
 
 
-# Test the function
-# print(calculate_self_consumption(1000, 1200))
+class EOSLoadInterpolator(SelfConsumptionProbabilityInterpolator, SingletonMixin):
+    def __init__(self) -> None:
+        if hasattr(self, "_initialized"):
+            return
+        filename = Path(__file__).parent.resolve() / ".." / "data" / "regular_grid_interpolator.pkl"
+        super().__init__(filename)
+
+
+# Initialize the Energy Management System, it is a singleton.
+eos_load_interpolator = EOSLoadInterpolator()
+
+
+def get_eos_load_interpolator() -> EOSLoadInterpolator:
+    return eos_load_interpolator