Funcs dict

adresses #449 (closed)

• adds the option to choose a function by passing a dict key to the saqc methods, wherever possible
• the dictionary of available functions is retrieved now from saqc.parsing.environ variables
• adds validation for function selection to the checking library
• moves helper functions from the lib.ts_operators module to saqc.func modules to avoid circular import
• moves the literal checking from funcs.tools to lib.checking to avoid circular imports

saqc/funcs/interpolation.py

@@ -104,7 +104,7 @@ class InterpolationMixin:
         self: "SaQC",
         field: str,
         window: str | int,
-        func: Callable[[pd.Series], float] = "median",
+        func: Callable[[pd.Series], float] | str = "median",
         center: bool = True,
         min_periods: int = 0,
         flag: float = UNFLAGGED,