Extend available Slicing Criterion Formats

[EagleoutIce]

E.g. one to access the second use of x in a line to have something better then 1:6 in x <- x as 1@x matches the first one!

The documentation should be updated accordingly.

[EagleoutIce]

The discussion resolved with the decision to focus on a function-call/query like syntax (hoping that this can be merged/unified with #1087 later).

In short: a syntax like get(foo, 'last')/get(foo, -1) is preferred (additionally to the existing formats).

[EagleoutIce]

We have agreed on a FlowR Search interface using a stream like processing approach.