This conditional determines, whether the scanned assignment
specified by the `assignment` key is going to be hidden in the
digital version of the document. It is by default, but some style
files (`/mu/sci`) switch this, so that the scanned assignment is
included in the digital version.

The output of \thesis@block@assignment and \thesis@@{assignment} now
depends on the value of the new \ifthesis@digital@ conditional.

Also removed a largely useless section from the guide.

When UPDATE_FAILED=true, failed tests will update the prototype PDFs
instead of terminating the procedure.

The tocdepth now uses the standard numbering (\part has tocdepth of -1).
Added a visual test suite.