Make all requirement nodes potentially assessable

[eric-intuitem]

Problem statement
I want to be able to make an assessment on a requirement node, even if the packager of the framework deemed it as not assessable. It can also be a way to add a global comment for a section, or a global proof.

Expected behavior
All requirement nodes shall be instantiated as requirement assessments when creating an audit.
The "assessable" attribute shall be copied, but it shall be possible to change it in the audit.