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updated installation instructions #2, CMake now handles compiler flags #1

Readme now contains installation instructions:
* Docker instructions
* Required software
* Ubuntu step-by-step manual installation (based on Dockerfile instructions)

CMake now handles compiler flags:
* build type is `Release` by default
* for a `Debug` build, warnings are enabled
* information on how to create different types of builds are added to the Readme

See merge request !1

Updated documentation build instructions (Sphinx and Breathe are installed by CMake during configure and probably not found during the first build)

  * use dunecontrol for all commands instead of fiddling with CMake in the build directory
  * some third-party version requirements seemed too strict
  * removed optional packages (METIS, parMETIS, Breathe, Sphinx)
  * removed Python VTK as suggested package. This is nearly impossible to build from source. People can use our VTK readers for post-processing (coming soon).
  * added section on cleaning build (delete build-cmake folder)
  * stressed that dunecontrol make install is optional (may not be desired by the user due to permissions / personal preference)

Added setup.py file for doc folder, so Sphinx and Breathe are installed at configure time now

Removed third-party Python packages from install_requires in setup.py files

Compiler flags are handled by CMake now. Default build type is Release, which automatically adds optimization flags, and equivalently for the Debug build type (cf. e.g. CMAKE_CXX_FLAGS_RELEASE).

-ftree-vectorize is already enabled by doing -O3 (https://gcc.gnu.org/projects/tree-ssa/vectorization.html)

For a Debug build, compiler warnings are enabled. For all other builds, optimization flags are invoked.