Add preferential use of functions to the prompt

[tischi]

@haesleinhuepf I thought it could be useful to tell bob to use functions whenever possible such that the code becomes more reusable both within the notebook and also for an eventual refactoring into something HPC executable.

Probably we should then also tell it to always add doc strings to the functions.

In terms of memory/token management for subsequent prompts we could then consider implement a summary that only keeps the function header and doc-string.