Document the analysis/eval pipeline

[raph-amiard]

After some time looking at the code, it is still not easy for me to understand some things, in particular:

• What are the different stages the wrp code goes through after being parsed.
• Where it is analyzed/evaluated

It would be cool if this was documented a little bit in the code

[QuentinOchem]

@raph-amiard agreed - this needs thorough documentation at this stage. I'll take it.

[QuentinOchem]

@raph-amiard see the entry point added under this issue - let me know if that's enough to get started. Closing for now, happy to re-open if needed.

(note that this is far from full documentation, but is intended to help jumping in).