TProcPool::Process(file, macro, tree, nMaxEntries)

A simple and lightweight way to do a parallel loop over
TTrees.

- support for multiple files as vector<string>,
  TFileCollection or TChain
- support for max number of processed entries
- automatic search for unnamed trees
- two different behaviours
   - nFiles < nWorkers: granularity at the event level
   - nFiles >= nWorkers: granularity at the file level

Signed-off-by: dpiparo <danilo.piparo@cern.ch>