Output functionals in CG discretizations

[sdrave]

This PR adds support for assembling arbitrary l2 output functionals (on the interior or the boundary) for cg discretizations:

• InstationaryProblem gets an additional functionals attribute to specify output functionals.
• Assembly of these functionals is only implemented for CG. FV is still missing.
• In pymor.operators.cg, the boundary treatment is moved to BoundaryL2ProductFunctionalP/Q1 which handles the assembly of dirichlet / neumann or arbitrary boundary l2 functionals. BoundaryDirichletFunctional is used for enforcing dirichlet constraints.

Additionally:

• A small fix for NumpyMatrixBasedOperator.H.
• Functions can now be multiplied, yielding a ProductFunction.

[sdrave]

FYI, I want this in 0.5 as a preparation for #454, #469.

[codecov]

Codecov Report

Merging #489 into master will increase coverage by 0.35%.
The diff coverage is 84.35%.

Impacted Files	Coverage Δ
src/pymor/operators/numpy.py	80.9% <0%> (ø)	⬆️
src/pymor/analyticalproblems/elliptic.py	100% <100%> (ø)	⬆️
src/pymor/functions/basic.py	87.39% <26.31%> (-11.63%)	⬇️
src/pymor/discretizers/fv.py	83.46% <50%> (-0.54%)	⬇️
src/pymor/discretizers/cg.py	86.06% <76.47%> (-2.01%)	⬇️
src/pymor/grids/interfaces.py	82.3% <85.71%> (-0.31%)	⬇️
src/pymor/operators/cg.py	87.64% <97.95%> (+5.43%)	⬆️
src/pymor/vectorarrays/block.py	92.46% <0%> (+0.68%)	⬆️
src/pymor/operators/constructions.py	84.18% <0%> (+1.48%)	⬆️
... and 3 more