use os.path.join to join path and filename

[andrewfowlie]

https://github.com/justinalsing/pydelfi/blob/5c4e7ede33f8055adab91435488454781c3861ea/pydelfi/delfi.py#L160-L161

It would be better to use https://docs.python.org/2/library/os.path.html#os.path.join and thus not assume that results_dir has a trailing separator. At present, without a trailing separtor, e.g., results_dir = 'here', the results_dir just becomes a prefix for filenames, not a directory.