tracker-turtle-reader: add support for comments inside triple patterns
Fixes the crash that happens when the ontology file contains comments at the middle a triple pattern. Before this commit, tracker only supports comments at the beginning or end of the triple pattern (either before the subject or after the dot). This commit consider comments the same as white spaces, can appear anywhere in the ontology file outside the string or IRIREF. So, skip_comments function is called each time advance_whitespace is called.