sort and map operations, and a better skimmer base module

op.sort sorts a collection by a function (ascending, so default sorting of physics objects would be by -pt); I included it here because it was in #12 (closed) and very closely related to the next one.

op.map gets an array (ROOT::VecOps::RVec) by applying a function on a collection - this is mostly useful for saving additional variables on skims (for further use in bamboo it is easier to simply apply the function on an element when needed).

There is also an update that brings the skimmer module beyond the proof-of-concept level: it is now possible to turn off the default copying of input branches (actually it is off by default, and can be enabled by passing --keepOriginalBranches to bambooRun), and adding branches is easier (see the new example for how to use it).

The rest are small fixes or additions to make the example work (e.g. op.static_cast because TBranch refuses to take a std::size_t - I don't see much other use for it, and some reorganisation)