New generated Stokes operators

See #246 (closed) for extended discussion

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Checklist:

• hyteg-operators submodule needs to be updated as soon as hyteg-operators!3 (merged) is merged
• fix unused variable warnings in generated operators
• move divergence, gradient, viscous directories out of the stokes directory and under hyteg_operators_composites instead
• fix issues for real_t = float (single precision builds)
• blending operators test

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New module hyteg_operators_composites

This MR adds a new module hyteg_operators_composites that contains composite operators that are built from generated operators.

For now, three variants of Stokes operators have been added:

• P2P1StokesConstantOperator
• P2P1StokesEpsilonOperator
• P2P1StokesFullOperator

including the necessary viscous, divergence and gradient operators plus their IcosahedralShellMap blending versions.

The new module also comes with a short README.

Testing

The MR extends and modifies

• P2P1ElementwiseStokesOperatorTest (Stokes constant, 2D + 3D)
• P2P1ElementwiseStokesOperatorTest (full Stokes w/ blending, 3D)
• ElementwiseEpsilonMinResConvergenceTest (Stokes epsilon, 2D + 3D)
• DivergenceOperatorTest (divergence, 2D + 3D)
• ViscousOperatorsTest (epsilon + full viscous block, 2D )

to test the new composites.

At least the 2D versions of all non-blending Stokes operators are now tested to some extent.