Improve handling of complex queries

[severin-lemaignan]

A query like:

Searching [u'?concept'] in models frozenset([u'SPEAKER']) matching:
    - (u'?concept', u'rdf:type', u'Car')
    - (u'?uSKTE', u'rdf:type', u'Brother')
    - (u'?caBkj', u'rdfs:label', u'"Danny"')
    - (u'?uSKTE', u'belongsTo', u'?caBkj')
    - (u'?concept', u'belongsTo', u'?uSKTE')

fails with this error message:

Can not handle pattern (u'?uSKTE', u'belongsTo', u'?caBkj') with requested variable ?concept.

because minimalkb does not yet handle queries where one statement contains 2 variables that are not the requested output (ie (u'?uSKTE', u'belongsTo', u'?caBkj') contains 2 variables that are different from ?concept)