Warn if no degree query function is set
Warn if no degree query function is set

The idea is to be able to use all filters with their default settings
without additional setup. In this case this was not fulfilled because
the query function was required.

VisualDataWeb/WebVOWL,leshek-pawlak/WebVOWL,leshek-pawlak/WebVOWL,MissLoveWu/webvowl,MissLoveWu/webvowl,VisualDataWeb/WebVOWL