Improve `get_io()`

[apdavison]

Since the same filename extension may be used by multiple formats (this is particularly true for .h5 and .txt), get_io() should be more sophisticated, e.g. try all possible IOs in turn until one works.

[zm711]

Fixed by combo of #1190 (list_candidate_ios-function which returns all possible ios so the user can decide for the correct one) and #1297 (get_io functionality improved and tries to return the most likely ios based on counting). Reopen if you were hoping for something different than these functionalities.