Support Gauss points in elliptic DG schemes

[nilsvu]

Proposed changes

Upgrade instructions

In elliptic executables add the Discretization.DiscontinuousGalerkin.Quadrature option. Set it to GaussLobatto to keep the previous behavior.

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[kidder]

@nilsvu this needs a rebase

[nilsvu]

Rebased.