`hipo-add` should give a clear error message when used on data files

Created by: c-dilks

On data files (not HIPO files filled with histograms), running:

hipo-add -o out.hipo in1.hipo in2.hipo

Outputs many lines repeating

(warning) :>>> no node present with group = 21, item = 7 positoin = -1
[hipo node] --> error : (1,1) requested type=INT has type=FLOAT
[hipo node] --> error : (1,1) requested type=INT has type=FLOAT
[hipo node] --> error : (1,1) requested type=INT has type=FLOAT
[hipo node] --> error : (1,1) requested type=INT has type=FLOAT

and eventually an empty HIPO file.

This is actually a groot problem, since hipo-add wraps org.jlab.groot.data.TDirectory. Note that for data files, run instead:

hipo-utils -merge -o out.hipo in1.hipo in2.hipo

It would be helpful for those of us who forget the difference between hipo-add and hipo-utils -merge to have better error reporting or usage guidance.