TrigValTools: speedup chainDump script

The chainDump.py script (used in all trigger ART tests) was taking about 1 minute to dump the YAML information of all trigger chains. Re-write the code to avoid the nested loops over chains and all chain steps. Now the script takes about 8 seconds for the LS2_v1 menu.

This also fixes a bug in the script extracting the step counts for chains that start with the same string as another chain. Reference files are updated accordingly.