Allow select expressions with multiple tokens after a tree selector

[SanderMertens]

Currently in the select, the tree operator // will allow a user to select a subtree of an object recursively. A user can for example request all objects under corto using the following expression: corto//*.

Not all usecases are covered yet. A tree operator that is followed by a more complex expression like foo//bar/* will not work. This is because the select first evaluates the entire expression, and then selects either a scope or tree based on the last selector token (/ or //).

The select has to be modified so that when it encounters a //, it will immediately walk a tree, and execute the remainder of the expression for every returned object of the tree walk.