Replace std::valarray with Eigen::VectorXd

Dmitri Naumovrequested to merge
endJunction/ogs:ReplaceValarray into master

Replace std::valarray with Eigen::VectorXd

For consistency mainly, but also save one instantiation of the shape functions. Also the valarray is kind of obsolete and Eigen provides equivalent substitute.

  1. [-] Feature description was added to the changelog
  2. Tests covering your feature were added? Existing tests
  3. Any new feature or behaviour change was documented?

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