Adding `parse_data_key_file` to `parsing` module.

Adding function for parsig "data-key.txt" files output by `ABCTeam`.
Adding test for function to `test_parsing.py`.
Updating `dmc.py` script.

Adding `--data-key-path` option to allow user to specify observed
summary statistics generated by a previous run of the script. In
combination with `--start-from`, it can be used to resume a run.