mfem: Errors in DGTraceIntegrator Documentation & Examples

I had been using the Doxygen documentation and there is an error for the BilinearFormIntegrator::DGTraceIntegrator. The integrator is actually implemented as (alpha/2) < rho_u (u.n) {v},[w] > + beta < rho_u |u.n| [v],[w] >. The documentation omits the factor of 0.5. Note that the documentation is correct for the closely related LinearFormIntegrator::BoundaryFlowIntegrator.

Tangentially related, the use of DGTraceIntegrator in ex9.cpp is incorrect. The factors should be 1.0 and 0.5 near here. Currently, the example flips the sign for one of the factors but this error is not observed because the inflow condition is set to zero.

I can create a PR for these changes if interested.