[pvtu2vtu] Corrected output file name

As titled.

The problem is that the output file path is extracted from the input string but the file name is not. That means if the output file name is input as path/output.vtu, the output file name is set as path/path/output.vtu by BaseLib::joinPaths(file_path, output_arg.getValue())). Actually there is no need to extract the path from the input string for output file name.

Thanks @FZill for reporting this bug.

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behavior change was documented?