WIP: Introduce PeriodicBasis

Oliver Sanderrequested to merge
introduce-periodic-basis into master

PeriodicBasis is a meta basis---a basis that is parametrized with another basis. In a PeriodicBasis, global degrees of freedom of the host basis can be grouped into equivalence classes, which are then treated as single global degrees of freedom by the PeriodicBasis. This allows, in particular, to implement periodic boundary conditions for discretizations without intersection integrals.

I would like your opinion on two issues:

  • Is this approach worthwhile at all? It doesn't cover all cases, but when it works it is very convenient.
  • Should this be in dune-functions rather than dune-fufem?

TODOS:

  • Make it a real meta basis. Currently, first-order Lagrange elements are hard-wired.
  • Allow to use this in a power basis.

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