Fixes PETSc dependencies in DiagonalNonConstantOperator and templatisation

All matrix setup stuff in the operator is in some form PETSc dependent.
It now gets wrapped into conditional compilation checking for the
HYTEG_BUILD_WITH_PETSC macro.

Additionally commit streamlines the template instantiation/specialisation
for the externaDiagonalAssembly() functions.

Also fixes WALBERLA_ASSERT_EQUAL vs WALBERLA_CHECK_EQUAL issue in test
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