union filters: some combinations don't yeld the expected result

[hotzeplotz]

initial bug for the feature implemented in #38: combining multiple union filters when one of them is based on [0,n] possible values per row (e.g. weapon type: more than one can apply to a single event) and another on [0,1] values (non-overlapping geo regions) does not yeld the expected result. the bars on the filter for the dimension with [0,n] possible values displays a higher count than what is eventually selected.