Preparations for whereClause

[Qup42]

There were still some preparations needed before the whereClause. I have gathered them here. Related to solutionModifier

• extract GroupKey, LimitOffsetClause, OrderKey and SparqlFilter from ParsedQuery
• create SolutionModifiers struct
• parse havingClause and solutionModifier with ANTLR

Others:

• parse triplesBlock with ANTLR

TODO:

• [x] parse filterR with ANTLR
• [ ] Tests for new SolutionModifier Visitor functions