constructGlobalToFlatLocalDofsMappingVectors takes now dof weights into account.
Extended the R-T space to allow DOF restriction to edges and/or elements
belonging to a grid segment.
Updated test suite of R-T space to match the recent interface changes.
Updated docs of PiecewiseLinearContinuousScalarSpace.
maxwell_dirichlet.py now uses a preconditioner.

Without the preconditioner, the solver didn't actually converge.