[Bugfix][ConvectionDiffusionDG] Honor absent boundary conditions

The ConvectionDiffusionDG operator has a boundary condition type 'None'
to indicate the absence of standard boundary conditions (e.g. for
coupling with Darcy).

This special condition was not handled correctly in the operator, which
always assembled some type of boundary condition.
[Documentation] Clarify PDELab version and dependency version requirements.
Merge branch 'releases/2.0'

Forward-port bugfix.

* releases/2.0:
  [Bugfix][ConvectionDiffusionDG] Honor absent boundary conditions