nx.tcl: - set multiplicty for mixins and filters by default from 1..n to 0..n to avoid error messages, when e.g. empty mixin lists are configured. Setting emtpy mixin-lists is strictly speaking not necessary, but this eases the code in cases the lists are sometimes empty.
- Replaced errorneous check using the object-specific rather than the class-specific options in the query mode of ::nsf::relation with class-filter. This prevented [::nsf::relation /cls/ class-filter] from returning any filter info. As a consequence, the incremental interface of the filter slot failed (e.g., [/cls/ filter add aFilterProc] effectively reset the class-filter cmd list instead of adding to it). Thx are due to Markus Moldaschl for reporting the issue. - Aligning class-filter and object-filter behaviour on errors during filter additions to the state-preserving behaviour of object-mixin and class-mixin. - Adding basic tests on object-filter/class-filter (::nsf::relation, filter slot interface) and specific ones to cover the above issues.