Update definition of a triple patterns and property path patterns for triple term patterns

[afs]

... I notice that the definition of a triple pattern needs to be changed! By the current definition, the object component may be either a variable or an RDF term. While the latter includes triple terms (even nested triple terms), it does not include "triple term patterns" (triple terms with variables). The definition of a property path pattern has the same issue.

Originally posted by @hartig in #53