Make combinations and slices behave more consistently

A few things that currently don't work, but should:

• passing a combination as input to a range function. Example from @fbury : ll = op.combine(...)[0]; minDR = op.rng_min(ll, lambda l : op.deltaR(obj.p4, l.p4))
• for l in ll (in python)
• op.combine(container[:X], ...) (I'm not entirely sure this was it, but I had to work around something like this in the H->4l open data example)