Improving memory management in the radiation dmg code

This MR does not aim to improve the speed of the code, but it does three other things:

• Deletes/closes files and histograms before event loop, once these are converted
• Adds a protection to check if the correct files/histograms are read
• Fixes one instance when the wrong type was read (TH3 vs TH2)

The results were cross-checked using diff-root wrt the code in master and no differences were found

cc @bnachman