Output file or directory in rsmpredict

Currently, `rsmpredict` supports an undocumented option of specifying an output directory instead of file if the output_file does not have a `.csv` or `.xlsx` extension. However, there are several inconsistencies:

1. This option is not documented so the docstring is inaccurate.
2. The output file format is controlled by the `file_format` setting in the `rsmpredict` configuration file and the extension of the specified file is totally ignored, if specified.
3. The directory bit is untested in addition to being undocumented.
4. The `.tsv` file format is not represented in the check that determines whether the output is a file or a directory.

I think a reasonable solution would be to:
1. Get rid of the directory output option entirely.
2. Make it so that the output argument is called `output_prefix` with the file format specified in the configuration file overriding the file format on the command line and an appropriate warning generated.