`root_numpy` dependency not documented, not easy to install

[jllanfranchi]

Also, apparently specific version(s) of ROOT and/or numpy might be required to get root_numpy to install. Please add info on which version(s) are "legal" to INSTALL.md and make some note of the dependency in the rooutils.py file EDIT: also in stages/unfold/roounfold.py). (I tried a quick pip install root_numpy and it failed asking for ROOTSYS, so if there is also a recipe for installing this, that'd be helpful.)

[jllanfranchi]

As an alternative, could the two functions that depend on root_numpy in rooutils.py simply be replaced with native ROOT implementations? I could be wrong, but it seems odd that you need an external dependency just to make 1D or 2D histograms, I'd think that ROOT's python/numpy interoperability (pyroot or whatever they call it when you import from ROOT.py) could do this by itself...

[LeanderFischer]

Also rooutils.py is gone, right, so closable, too..