Simplify extending the catalog

[veenstrajelmer]

If a user wants to add a property to the catalog, this would be the way to go:

import ddlpy
from ddlpy.ddlpy import ENDPOINTS
station_code = "ossenisse"

endpoint = ENDPOINTS["collect_catalogue"]
request = endpoint["request"]
catalog_filter = list(request['CatalogusFilter'].keys())
catalog_filter.append("Bemonsteringshoogten") # This is probably invalid input

locations = ddlpy.locations(catalog_filter=catalog_filter)
print(locations)

This is a bit cumbersome. Consider making the default catalog_filter accessible or document this somewhere.

Good to know, there is no error given on invalid inputs, this is reported in Rijkswaterstaat/WaterWebservices#18.