relicensing

deal.II relicensing effort

With this commit the deal.II project has adopted the following new
licensing rules outlined in LICENSE.md and CONTRIBUTING.md:

  As a contributor to this project, you agree that all of your
  contributions to deal.II (a) be governed by the Developer Certificate
  of Origin version 1.1 and (b) be dual-licensed under the terms of the
  Apache License 2.0 with LLVM Exception and the GNU Lesser General
  Public License v2.1 or later; see LICENSE.md for details. The deal.II
  project does not require copyright assignments for contributions. This
  means that the copyright for code contributions in the deal.II project
  is held by its respective contributors.
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