Add 'ZIP' variable ordering to Picotrav

[Steffan Sølvsten]

Another simple variable ordering that would be of interest is a zipped ordering. This is specifically of interest for the circuits with two inputs, i.e. most of the arithmetic circuits of EPFL. Assuming the input is given as a[0], a[1], ..., a[n-1], b[0], b[1], ..., b[n-1] we would like to derive the ordering a[0], b[0], a[1], b[1], ..., a[n-1], b[n-1].

This can be done with a simple for-loop with pointers starting at the first and middle-most element.