support parameter subelement accessors

[petri]

Make more advanced parametrization possible by using a slash to indicate a subelement of a parameter data structure, thus: $parameter/subelement, or $parameter/0. The first case would return a subelement of a mapping, by key, and the latter of a sequence, by index (starting from zero).

For implementation, it may be sufficient to just split the parameter at slashes and iterate over them and the parameter data structure.