Decide on minimum required versions for dependencies after 4.1

[RudolfWeeber]

[jngrad]

• Python (currently >= 3.3), Cython (currently >= 0.23)
• CMake: discussion in Bump required cmake version to 3.10 #3090
• boost: versions below 1.67 make the CMake logic convoluted, dockerfiles require patches
• python packages: numpy, scipy, pylint, vtk, etc. should be written to requirements.txt
• MPI via OpenMPI: depending on the OS, a different minimum version is required

[jngrad]

Concerning version numbers for python dependencies, the version numbers of the corresponding packages in the Ubuntu 16.04 repository (which we currently support in the Clang 6 image) would offer a conservative estimate, unless stated otherwise, for example pint is pinned to a higher version due to numpy-related compatibility bugs in the version available on Ubuntu.

[hmenke]

Also #1644 requires at least Boost 1.65.1, because it's now using Boost.Spirit X3 which is relatively new.

[jngrad]

Update: a discussion took place in es meeting 2019-11-26
We'll probably need to increase the boost requirements in two steps: first 1.58, which we have to support until the Ubuntu 20.04 LTS release (we need to support Ubuntu 16.04 LTS until then), then 1.65. Python versions 3.3 and 3.4 are only supported on CentOS SCLo.

[jngrad]

Once we get rid of clang:6.0 in CI (which uses 1.58), we should be able to bump to 1.65, which is used by osx in CI. We'll then be able to simplify CMake and remove all #if BOOST_VERSION. Python itself and Python packages will need to be kept in sync with NEP 29 (#3421), which should not be difficult considering we support Ubuntu LTS versions for 3 years.

[RudolfWeeber]

Based on Ubuntu 20:

• Remove Cuda 9 asupport
• Cuda 11 with coverage in every build
• CMake 3.16
• Cython 0.29
• Python 3.6
• Boost 1.66

Drop Centos build. Clusters use custom compilers and libraries via modules anyway.