Pull in fixes from release branch

* releases/2.0:
  [Release] Update version information and release notes for 2.0.0
  [Documentation] Mention dune-typetree source package download location
  [Buildsystem] Clean up build in boilerplateturial
  [Boilerplate] Make sure to use correct geometry type for all grids
  [Git] Add build-cmake to .gitignore.
  [Documentation] Remove unecessary LaTeX packages.
  [Documentation] Fix introduction of 2.0 release in RELEASE_NOTES
  [Documentation] Update release notes and bump version to 2.0.0-rc2
  [Dependencies] Bump version requirements of the core modules to 2.3.1
  [Documentation] Remove notice to use the release branch of the core modules - 2.3.1 has been released
  [Cleanup] Fix trailing whitespace in Howto TeX files
  [Howto] Fix compilation problem in beamer mode
  [Bugfix][Howto] Work around abug in the interaction between KOMAScript and headings
  [Howto] Fix warnings about deprecated LaTeX macros
  [Release] Update version information for dune-pdelab-howto 2.0.0-rc1

Conflicts:
	README
	dune.module