introduce tokenizer modules

This adds the boilerplate for selecting configurable tokenizers.
A tokenizer can be chosen at import time and will then install
itself such that it is fixed for the given database import even
when the software itself is updated.

The legacy tokenizer implements Nominatim's traditional algorithms.

test/python/conftest.py

@@ -144,6 +144,11 @@ def tmp_phplib_dir():
 
         yield Path(phpdir)
 
+
+@pytest.fixture
+def property_table(table_factory):
+    table_factory('nominatim_properties', 'property TEXT, value TEXT')
+
 @pytest.fixture
 def status_table(temp_db_conn):
     """ Create an empty version of the status table and