COMBINED_MERGE.LOG for each pippin stage

[RickKessler]

for complex pippin jobs it is difficult to check statistics of sims/lcfit/bbc because we have to drill down into each output directory. It would be really useful to combined the MERGE logs into one file per pippin task; i.e., scoop up
ls 2_LCFIT/FIT_*/output/MERGE.LOG
and paste the contents into 2_LCFIT/MERGE_COMBINED.LOG along with a comment header above each file-content so that we can visually associate each pippin-directory with MERGE.LOG contents. Such a file is not yaml-compliant, but the sole purpose is human readability.