CMake Modernization

The goal is to modernize CMake so that two things are possible:

Cmake native usage of dune

cmake_minimum_required(VERSION 3.16)
project("dune-foo" CXX)

# find dune-bar project: exports the target Dune::Bar
find_package(dune-bar)

# create a target library with native cmake
add_library(dune-foo INTERFACE)

# link to dune targets without need of dune-isms (e.g., custom generation of config files)
target_link_libraries(dune-foo INTERFACE Dune::Bar)

Superbuilds

cmake_minimum_required(VERSION 3.13)
project(dune VERSION 2.9 LANGUAGES CXX)

foreach(module dune_modules dune-common dune-geometry dune-localfunctions dune-uggrid dune-grid dune-istl)
  add_subdirectory(${module})
endforeach()

Issues and MR in this Milestone contribute one way or another to these goals.