Tree.Draw: fix 2D, 3D and 4D drawing and no longer support drawing lists of expressions
more thorough Tree.Draw tests
context.set_directory: do nothing if object has no SetDirectory
Tree.Draw: only allow drawing into a TH1 if hist is specified
new create_hist arg in Tree.Draw to override ROOT's default behaviour and fill hist in all cases
Merge pull request #388 from ndawe/master

[MRG] Tree.Draw improvements