WIP: Semilinear functional

Open lh1887 requested to merge feature/semilinearfunctional into master

This MR implements a semilinear functional of the form

J(u) = \sum_i \omega_i \varphi(u_i)

where the index i corresponds to the scalar entries (but the vectors should be able to be arbitrarily blocked). The function phi needs to be supplied by the user and has to comply to a (not yet fixed) reasonable interface.

This functional explicitly does not carry any quadratic part of the functionals many of us would consider. The idea is to integrate this part via using SumFunctional and (e.g.) QuadraticFunctional as needed.

This is still work in progress, there are several things remaining, among them:

  • tests
  • examples (say e.g. Allen-Cahn with different potentials)
  • documentation
  • clean up (names, files, ...)
  • many things can be done more efficiently
  • bug fixing

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