Defining codes/executables for ShellTask

[GeigerJ2]

For future reference:

src: ./statistics_foo
command: "./statistics.py"

with shebang, or, without:

src: ./statistics_foo
command: "python3 ./statistics.py"

with correct Python in the $PATH in the uenv.

• Also being able to provide the labels of already registered AiiDA Code instances?
• Possibly copy over directory ourselves before, rather than creating a PortableCode (@agoscinski).
• Make sure we don't create too many Code instances (e.g., automatically with aiida-shell), neither that we accidentally use the same Code for multiple tasks, where it shouldn't be used (e.g., because an instance is loaded from the DB)