fix: rename hipo-add to hipo-merge-histograms and clarify usage

[c-dilks]

Resolves #323

[baltzell]

maybe best to just remove it altogether?

[c-dilks]

maybe best to just remove it altogether?

That was my original idea, but hipo-utils -merge does not actually add histograms (even though it returns exit code 0). On the other hand, hipo-add / hipo-merge-histograms does. So we have:

• hipo-utils -merge for combining data files (e.g., DST files)
• hipo-merge-histograms for combining files with histograms

On the other hand, ROOT's hadd does both of these, which is why this behavior confused me.

A better solution would be to have hipo-utils -merge add the histograms correctly, but that's an upstream issue (org.jlab.jnp.hipo4.utils.HipoUtilities, cc @gavalian)

[gavalian]

Merge is a function of HIPO library, which just merges events. However, the hadd is a function of GROOT which has to read and interpret histogram classes and add the content. The HIPO library does not have histogram implementations and can not read and parse the content. That's why this functionality is kept separate.