Add open_file utility function

It might be nice to include the `open_file` function from https://github.com/goerz/qdynpylib/blob/181d69bed726fac789af3d66ad1f4f881d09450c/QDYN/io.py#L24 in `utils`, and to use it anywhere `with open(...` is used right now. It adds quite a lot of flexibility for I/O operations.

Very low priority.