revise dependencies, add dependency diagram

[VladimirAlexiev]

Here are the current module dependencies after taking into account #112:

• can we eliminate the cyclical dependency between check_method and compliance_verification_report?
• permitting_stages is not imported anywhere, maybe we should import it to aec3po?
• are there other non-imported modules?
• aec3po redundantly imports document, evidence, check_method. Maybe remove these imports?
  • On the other hand, the list of imports is the first thing one sees on the home page, so maybe it's better to list ALL modules there? I prefer this variant
• Add a dependency diagram to the main doc https://ci.mines-stetienne.fr/aec3po/. The diagram is made with graphviz or plantuml; the source is in the first link