I/O: Runner file handling

TODO:

• Explain output tuple, root attribute.

Default runner behaviour

While custom Styx runners allow users to implement file I/O handling any way they want, the 'default' runners (LocalRunner as well as DockerRunner and SingularityRunner) work very similarly by default.

They create a directory (default path styx_temp/ in the current working directory) in which a folder gets created for every Styx function call. These will look like this:

79474bd248c4b2f1_5_bet/
^^^^^^^^^^^^^^^^ ^ ^^^
|                | |
|                | `-- interface name (FSL BET)
|                `---- number of execution (5th)
`--------------------- unique random hash

Every time you create a new runner a new random hash will be generated. This ensures paths created are unique and avoid conflict with previous executions of your pipeline. The hash is followed by a counting number which chronologically will count up by one for every Styx function call. This makes the folder sortable. Lastly they contain the Styx function name to be human readable.