Allow user to force writing outputs into existing output directory

[jpalmer37]

Currently, fluviewer will fail if you give it the command line parameter --outdir . because it already exists. This was implemented to prevent overwrites of existing result folders.

Since the overwrite check is a safe design choice, I don't necessarily want to remove it. I am considering whether we could solely skip an overwrite check if outdir is the present working dir (.) and check any other outputs like normal. Open to suggestions.

[dfornika]

How about this? We add a --force flag that allows the user to specify that they want to proceed with analysis even if the outdir already exists.

[dfornika]

Fixed in #30