Jan Nikl

Well, it originates from [my fork of MFEM](https://github.com/najlkin/mfem/) like many other things, so I definitely used that somewhere in past. Currently, I am relying on this branch in the HDG...

@psocratis , maybe look at `ex5-hdg.cpp` in `darcy-hdg-dev`. I am using new `DGNormalTraceIntegrator` there, but it is very similar. Actually, maybe it would be a good idea to include it...

@psocratis that is a good point 😄 , it seems the BCs are not handled generally enough. I should return to that in the HDG branch. Anyway, that is not...

Folks, I have decided to remove the mixed elements methods in `NonlinearForm`, because there is no matching `MixedNonlinearForm`, so it perhaps should a part of a bigger effort to add...

I just removed virtual and added override as there is now consensus on it (see https://github.com/mfem/mfem/pull/4399 ) 😉 .

Added support of integral finite elements to `DGTraceIntegrator` following #4088 .

Ok, I see, it is too big, so no one wants to take it 😁 . I will split it, keeping this one only for the classical boundary/face integration.

Ok, ready for review again 😉 .

> @najlkin, would it make sense to include these new functions in one of the CI tests? Or do you have some driver code that can be used to test...

@v-dobrev , there is the method `Hybridization::AssembleBdrMatrix()`, so it seems there was an intention to enable boundary terms, but you have to do the integration manually. This PR adds the...