DirichletBC: Use specialization for Constraints instead of general case

Merged Müller, Felix requested to merge issue/dirichletbc_use_constraints_specialization into master

src/amdis/DirichletBC.inc.hpp calls

34: auto columns = Constraints<Mat>::dirichletBC(matrix, dirichletNodes_);

There are specializations for this in the LinAlg implementations, yet only the generic implementation is included in the file.

This MR replaces the generic include <amdis/linearalgebra/Constraints.hpp> with the specialization in <amdis/linearalgebra/*/Constraints.hpp>. This is already done in src/amdis/PeriodicBC.inc.hpp.

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