Build system changes in the python branch

[Jean-Noël Grad]

Dear users,

We would like to share with you some important changes to the way the 
development version of ESPResSo is configured and built.

As discussed in the community call from October 13, 2022, we are working 
on integrating more external libraries into ESPResSo, such as waLBerla 
for hydrodynamics and librepa+kdpart for MD load balancing. We opted for 
the CMake FetchContent method, which allows us to finely tune the way 
these libraries are compiled and integrated within our codebase. In the 
process of doing so, we noticed imperfections in our build system that 
made it difficult to integrate new libraries [1], or to use ESPResSo as 
a library in a parent project. In particular, CMake flags such as 
WITH_FFTW were inherited by included projects, which could cause them to 
build components that were not needed in ESPResSo. This behavior would 
unnecessarily increase the build time, and in a few cases, force users 
to install extra dependencies to complete the build. The latter is 
problematic for clusters users, who typically have access to a 
restricted set of dependencies and cannot easily install new packages.

CMake options name conflicts are commonly resolved by adding a unique, 
project-specific prefix on all CMake options. While we worked with 
external libraries maintainers to rename their CMake options, we 
obviously needed to rename ESPResSo CMake options too. As such, on the 
python branch of ESPResSo, a prefix `ESPRESSO_BUILD_` was added to all 
options that control compilation, and a prefix `ESPRESSO_` was added to 
the other options [2]. The new option names can be found in the user 
guide, section 2.4.4. "Options and Variables" [3]. If you accidentally 
use the old options when building the development branch of ESPResSo, or 
the new options when building older ESPResSo releases, CMake will print 
a warning and list which options were not recognized.

We also took this opportunity to modernize the ESPResSo codebase by 
requiring C++17 and CUDA 11, as well as enforcing minimal versions for 
compilers and MPI libraries. These changes helped us simplify the CMake 
logic and the C++ core greatly, and we expect that contributing new 
features to ESPResSo will lead to a smoother experience for first-time 
contributors.

We will continue supporting C++14 and CUDA 10 in the 4.2 line of 
ESPResSo, and a bugfix release is currently in preparation [4].

Best regards,
Jean-Noël Grad

Links:
[1] https://github.com/espressomd/espresso/issues/4618
[2] https://github.com/espressomd/espresso/pull/4612
[3] 
https://espressomd.github.io/doc/installation.html#options-and-variables
[4] https://github.com/espressomd/espresso/wiki/es-421-release-notes