Add merge_routings program, search all placemen...

Add `merge_routings` program.
Usage:

    merge_routings.py [-h] [-a | -f]
                      merged_output_path input_files [input_files ...]

    Merge placement results HDF files.

    positional arguments:
      merged_output_path
      input_files

    optional arguments:
      -h, --help            show this help message and exit
      -a, --append
      -f, --force_overwrite

Fix `route_from_hdf` to search all `placements` tables, do not stop searching
after the first `placements` table found.
Add dump_round_robin_placement_sha1s program

Usage:

    dump_round_robin_placement_sha1s.py [-h] placement_tables_hdf_file
    dump_round_robin_placement_sha1s.py: error: too few arguments