add convenience import

Author: sprivite

phenex/filters/__init__.py

@@ -1,5 +1,8 @@
-from .relative_time_range_filter import RelativeTimeRangeFilter
+from .categorical_filter import CategoricalFilter
+from .codelist_filter import CodelistFilter
 from .date_range_filter import DateRangeFilter
+from .relative_time_range_filter import RelativeTimeRangeFilter
+from .value_filter import ValueFilter
 from .value import (
     GreaterThan,
     GreaterThanOrEqualTo,