Add sh:SelectExpression to SHACL-SPARQL

[HolgerKnublauch]

The Core spec (assuming #222 gets merged in) contains a reference to a node expression function sh:SelectExpression, in an example, with a reference to SHACL-SPARQL where it is supposedly defined. This actually should happen now, as the first non-trivial node expression type.

I believe defining it is straight-forward, but the real work here is to cover it with test cases, because this will be the first time that node expressions will be making a real difference to features such as sh:targetNode and sh:path. This also means that engines will need to add this extra flexibility.

[ajnelson-nist]

Cross-linked #297 .