introduce generic YAML config loader

Adds a function to the Configuration class to load a YAML
file. This means that searching for the file is generalised
and works the same now for all configuration files. Changes
the search logic, so that it is always possible to have a
custom version of the configuration file in the project
directory.

Move ICU tokenizer to use new load function.

nominatim/tokenizer/icu_tokenizer.py

@@ -8,7 +8,6 @@ import json
 import logging
 import re
 from textwrap import dedent
-from pathlib import Path
 
 from nominatim.db.connection import connect
 from nominatim.db.properties import set_property, get_property
@@ -49,12 +48,8 @@ class LegacyICUTokenizer(AbstractTokenizer):
             This copies all necessary data in the project directory to make
             sure the tokenizer remains stable even over updates.
         """
-        if config.TOKENIZER_CONFIG:
-            cfgfile = Path(config.TOKENIZER_CONFIG)
-        else:
-            cfgfile = config.config_dir / 'icu_tokenizer.yaml'
-
-        loader = ICURuleLoader(cfgfile)
+        loader = ICURuleLoader(config.load_sub_configuration('icu_tokenizer.yaml',
+                                              config='TOKENIZER_CONFIG'))
         self.naming_rules = ICUNameProcessorRules(loader=loader)
         self.term_normalization = config.TERM_NORMALIZATION
         self.max_word_frequency = config.MAX_WORD_FREQUENCY