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/scores.py

@@ -125,7 +125,7 @@ def _univarScoring(
     min_periods
         Minimum number of valid meassurements in a scoring window, to consider the resulting score valid.
     """
-    validateFuncSelection(model_func, allow_operator_str=True)
+    validateFuncSelection("model_func", model_func, allow_operator_str=True)
     if isinstance(model_func, str):
         model_func = ENV_OPERATORS[model_func]
     validateFuncSelection(norm_func, allow_operator_str=True)