[Tests] Remove broken and outdated test 'poisson'

For an equivalent example, look at testpoisson.
[testpoisson] Bring testpoisson up to date

- Use generik qkfem
- Use new matrix backend
- Remove unused typedefs
- Adjust to modified way of specifyin integration order of Poisson local
  operator
[Tests] Remove broken and outdated 'instationarypoisson'

Look at testinstationarypoisson instead.
[explicitlycoupledpoisson] Drag test into the present

While explicitlycoupledpoisson basically does the same thing as
testpoisson, it might still be useful, both as an example of how to do
stuff lower-level at the LA stage without using the backends from
PDELab, and as an example of how to port older code.

Fixing this required changes to a number of things

- Switch to generic QkLocalFiniteElementMap

- Use new, more efficient pattern construction by switching to new
  matrix backend; also need to pass an instance of the new backend to
  the GridOperator

- The constraints container picked up the GFS type as part of its type
  signature, so it needs to be obtained from the
  MultiDomainGridFunctionSpace

- The leaf GridFunctionSpaces must be named to make the new VTK output
  machinery happy

- The Poisson LocalOperator now takes the quadrature order as a runtime
  parameter

- There are no TypeBasedGridFunctionSubSpaces anymore; I doubt that they
  will come back due to the completely different way the underlying
  subspaces from PDELab work

- Output pattern statistics for tuning purposes

- When directly using ISTL components to construct the solver, the types
  and objects of the PDELab wrappers have to be unwrapped

- Use new, streamlined VTK output support
[Tests] Remove testassembler

testinstationarypoisson tests everything this test did, and more.
[Tests] Remove instationaryexplicitlycoupledpoisson

This thing is way too ancient to try porting it...
[Tests] Remove testvisitor

Another test that was nothing but a development helper...
[testmatrixconstruction] Port to new infrastructure

- Use unified QkLocalFiniteElementMap

- Remove unused typedefs

- We new have two component spaces with identical type, so cannot use
  TypeBasedSubProblem anymore. Switch to standard subproblem setup with
  indices instead.

- Adjust MultiDomainGridFunctionSpace typedef to include backend and
  ordering tags

- Fix Poisson operator quadrature order handling

- Use new matrix backend with better pattern construction and output
  pattern statistics
[testmultidomaingridfunctionspace] Get this to mostly run again

Also ignore the binary.