Updating `ABCTeam`.

Updating `_write_keys` method of `ABCTeam` to write the relative
paths to model configs and observed data, rather than the full
paths.

Also initiating `i = -1` at the beginning of the
`_prior_summary_worker_iter` method, so that `i` is initiated even if
the number of prior batches is zero.
Updating `ABCTeam`.

Initiating `i = -1` at the beginning of the `_prior_worker_iter` method,
so that `i` is initiated even if the number of remaining prior batches
is zero.
Updating `dmc.py` script.

Changing the logging of info to the `pymsbayes-info.txt` file.
Specifically, I am changing the writing of all config and observed stats
files to write relatvie paths, rather than full paths.