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

CHANGELOG.md

@@ -16,7 +16,6 @@ SPDX-License-Identifier: GPL-3.0-or-later
 [List of commits](https://git.ufz.de/rdm-software/saqc/-/compare/v2.4.1...v2.5.0)
 ### Added
 - Function selection via strings for most function-expecting parameters
-- WMO standard aggregations `trueDailyMean` and `climatologicalMean`
 - `SaQC.plot`:
   - enable multivariate plots
   - keyword `plot_kwargs` to pass matplotlib related arguments