Fix to names of PS-POH column headers + Allow 2S-SEH and PS-FEH tests to...

Hello,

I fixed some issues in the PS-POH results name definitions in data_description in modules/Hybrid.py. I also changed the function that recovers the ROOT file to accept a single ROOT file in the cases where two are suppossed to be there (2S-SEH and PS-FEH). In the case of the 2S-SEH, it is not fully certain yet that both files will be always created.