Ubuntu 20.04 distribution with clang-10 and gcc-9
Summary
This MR adds another ubuntu distribution with more recent clang compiler and other tool in newer versions
- clang: 10.0
- gcc: 9.3
- cmake: 3.16.3
For each compiler we add two c++ standard version, c++17 and c++20 as toolchain each with and without assert enabled. So, in total we have 8 toolchains:
- gcc-9 (c++17, c++20, c++17-noassert, c++20-noassert)
- clang-10 (c++17, c++20, c++17-noassert, c++20-noassert)
The dune-git images are now build with ubuntu-20.04 instead of ubuntu-18.04. The reason is that we will need a more recent cmake version in the near future.