Merge the dune-tnnmg-examples module into dune-tnnmg

See merge request !18

ParMetis does something weird with the mpi.h header and C linkage.
This leads to "conflicting declaration" errors for MPI methods.
As we are not using ParMetis anyway let's just disable it.

Yes, this is not particularly pretty, but the problem will go away
once we port away from dune-pdelab, which we plan on doing anyway.

It is very old, after all.  Unfortunately, it is the one that
is described in the documentation, but that documentation
will need to be rewritten from scratch anyway.

dune-pdelab does some 'extern C' magic with mpi.h which makes
the CI system fail.  Force correct inclusion of 'mpi.h'
by including it (almost) explicitly, up front.

Which should translate to 'with and without the examples'.

TrescaFrictionFunctional only exists for demonstration purposes,
and at this stage a failing test shouldn't block getting the
examples into dune-tnnmg.

It is used for the examples.

I am not really sure what this stuff is all about..

It is preferable to not have this in a separate module:  The code
gets forgotten and slowly rots...

Do not depend on ag common

See merge request !23

'ag-common' is the old name of dune-fufem, and has gone out of use
years ago.  And dune-tnnmg doesn't depend on dune-fufem anyway.

Thanks to Andreas Dedner for spotting this.

Feature/make old tnnmg compile again

See merge request !21

In order to still allow using the old solver, the old style
SumFunctional is now called VintageSumFunctional for transition

Add SumFunctionalConstrainedLinearization class

See merge request !20

Just as SumFunctional implements the sum of several independent
functionals, this new class SumFunctionalConstrainedLinearization
implements the sum of linearizations (or, equivalently, the
linearization of the sum).

This was only available using a compile time switch.
Since the old style TNNMG implementation was ajusted
to the new bisection interface, this is no longer needed.

This is needed, since the fastQuadratic option of the bisection will be
removed

There is stil it lot of code related to the old implementation.
To make it work, we need to re-add the old iteration step. This
is only done for a transition period and will be removed again,
once the new implementation works in the same generality.

Stop using deprecated class VirtualFunction

See merge request !19

Port bcqfconstrainedlinearization.hh to 'if constexpr'

See merge request !15

It is not supported by Dune 2.8 anymore.

Minor updates

See merge request !14

Otherwise use Functional::Vector

Merge branch 'feature/proper-generic-copyOrReference-in-BoxConstrainedQuadraticFunctional' into 'master'

Properly forward constructor to CopyOrReference members.

See merge request !11

Update CI config to clang-7

See merge request !13

The old image is no longer updatet