This corrects some definitions of the methods in the base condition
class to make things more obvious to developers implementing new
filters.
Previously if your filter wanted to use the default
`core/datafilter/filtertype` class, you still had to implement
`get_filter_class` to return `null`, since it was declared as abstract.
This change defines it as returning `null` by default, so this is no
longer necessary.
Also, this removes the default definitions for `get_condition_key` and
`build_query_from_functions`, and declares them abstract. Currently it
is necessary to override these to implement a functional filter so it
doesn't make sense to have a useless default definition.
This will not cause any breakages with existing filters. All filters
must already be defining the methods that are now abstract, otherwise
they will not function. Any filter that is now overriding
`get_filter_class` to return `null` will continue to work as before,
even though this is no longer necessary.
Mark Johnson
1613a98102