fix docu image

See merge request !345

elementwise vs constant op

See merge request !344

Mixed forms for P2-P1 Stokes with blending

Closes #126

See merge request !343

Comparison of local element matrix between HyTeG and FEniCS with
"challenging" affine blending looks good.

This is another step towards the implementation including blending. We test
that the results we get from our HyTeG form match those from the FEniCS form.
Next step will be to include the blending.

fix coverage reports

See merge request !340

3D stokes with blending

Closes #127

See merge request !341

Load balancing

See merge request !338

improve coverage; enable clang 10; change walberla module to relative path

See merge request !339

Objective was the reduction of global communication to a minimum.
The migration did not scale well above 10k processes in some experiments.

Major changes
- not only the primitive -> rank map, but also the number of
  of expected primitives after migration is required for migration
  (wrapped by new class MigrationInfo)
- added helper function that gathers the number of expected primitives
  if it cannot be easily calculated by the load balancing strategy
  (this method is expensive / scales badly, though)
- replaced global reduction of migration info by neighborhood
  communication, gathering the future ranks of neighbor primitives
  before the actual migration
- replaces global p2p message passing by using new PackageBufferSystem
  (no need to send empty messages to all processes anymore)
- renamed copyDistribution() to reverseDistribution() because it fits better,
  also massively refactored the code
- some minor test fixes

This buffer system is intended to allow communication if the sender
ranks are not knowns on the receiver side, but the number of packages
that are expected, is. Such functionality is not available in walberla.

Thus, this might be ported to the walberla core eventually.

Trilinos integration, Superman agglomeration

See merge request !336

use gcc:9 image for coverage job

See merge request !337