Add a node expression type for calling a node expression

[bergos]

It was previously mentioned that node expressions cannot use an IRI as an identifier because they would be interpreted as a constant term. To workaround that problem, a new node expression type that accepts the IRI of a node expression as the key parameter should be defined.

@afs proposed sh:call:

 [ sh:call my:nodeExpr_numTables ]

Since there doesn't seem to be a better place for it, we should consider making this a node expression type defined in the core specification.