Fix parsing datetime UserWarning

[amundmr]

When batch processing loads of EIS data using gamry-parser, the following user warning is always printed to stdout.

UserWarning: Parsing dates in %d.%m.%Y %H:%M:%S format when dayfirst=False (the default) was specified. Pass 'dayfirst=True' or specify a format to silence this warning. start_time = pd.to_datetime(

It originates from here.

I volunteer to fix it if assigned 👍

[bcliang]

@amundmr if you are still available, please do submit a PR. Ideally you could base off of type-hints

It has been a couple years since I have looked at this repo. I am sure there is a lot to cleanup.