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[parallel,lagrange] assemble matrix on AllPartition, but residual only in interior

Christian Engwer requested to merge bugfix/parallellagrange into releases/2.8

Currently operator and residual are assembled only in the interior. This breaks parallel computations with conforming elements. At the same time any operator evaluation (e.g. computing the residual) ends with a global sum, so that updates on different ranks are summed up.

There are different possible approaches to fix this.

  1. decomposition of DOFs
  • assign each DOF to a unique rank (e.g. lowest rank number)
  • assemble matrix & residual on AllPartition
  • residual is automatically consistent, no communication required
  • norms are only evaluated for DOFs owned by the rank
  1. decomposition of the
  • assemble matrix on AllPartition, operators only in interior
  • residual needs to be communicated (all-to-all) and summed up to have the complete residual.
  • as operators are evaluated only in the interior, the norm is automatically consistent

This is the first step to fix #171

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