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Add P1+B simplex bubble functions

Simon Praetorius requested to merge feature/simplex-bubble-functions into master

Summary

This MR adds a local finite-element representing linear Lagrange functions enriched with a single bubble-function b_T that is defined as the product of the linear basis functions.

The basis functions are implemented for simplex elements only in dimension 1,2, and 3.

Possible improvements

  • Make the local coefficients class also constexpr. This requires that the LocalKey is constexpr, see !263 (merged).
  • Better naming? "P1B" means linear functions + single bubble function. This is sometimes used in literature.
  • Document the normalization? The bubble functions are products of all linear basis functions scaled by (dim+1)^(dim+1) such that the evaluation in the barycenter gives the value 1.
Edited by Simon Praetorius

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