Fix file reading bug in #14

[ced4rtree]

See #14

I forgot to parameterize the check_pair_energy function in dpd_utils.py but failed to catch it since the function was reading from an old log file while I was testing. I added a test in this PR to check functionality of both creation & reading of the log files produced when a log file is specified.

[ced4rtree]

After checking this again, I'm wondering if log_file_name should really have a default value. It's only used internally and having a default value for file reading seems like an easy footgun.

[erjank]

I'm not clear on the proposed path forward. On one hand, "log.txt" is a great default filename, but if the worry is that we'll overwrite something that we'll need, then creating unique identifiers for each run and then cleaning them up (or not) seems like the sensible choice.

[ced4rtree]

The worry in my comment was about reading from the wrong data. Defaulting to overwriting files with a default filename is the status quo in FlowerMD, so I'm fine with keeping that (but not opposed to changing if that's desired). What I'm worried about in this case is if somebody has a log.txt in the working directory and calls check_pair_energy but forgets to supply the file name of the file they want to read from, it would lead to reading from the wrong log.