Add C++ compiler as dependency

[ajaust]

This mention a C++ compiler as dependency for building the bindings in the README. It was pointed out that installation of the bindings fails, if there is no C++ compiler installed.

Do we actually need MPI as a dependency as well?

[IshaanDesai]

Makes sense to have a note that a working C++ compiler needs to exist on the system. Regarding having MPI as a dependency, things are a bit unclear. In the bindings we import mpi4py but never really use it, so I am also unsure whether we really need MPI or even mpi4py as a dependency. @BenjaminRodenberg worked on #101 some time ago, which removes the MPI dependency altogether.

[BenjaminRodenberg]

If I understand this right, this PR is directly related to precice/precice#1225. Because it already caused problems that the C++ compiler is undocumented, I think it's a good idea to add a note in the documentation.

Finding a proper treatment for C++ dependencies of this package is a bigger issue. Checking for cython is something we are also missing currently, as far as I see it. But I am not sure whether we actually want to do this (see, for example https://github.com/pandas-dev/pandas/pull/9881/files#diff-60f61ab7a8d1910d86d9fda2261620314edcae5894d5aaa236b821c7256badd7).

Let's document the C++ compiler, but not change the actual implementation because this might cause more problems than it would actually solve.

[ajaust]

Yes, it is linked to the issue in the preCICE repository. I agree that that it should be enough to document the need for a C++ compiler and changing the whole installation process would be too much at the moment.

I think the main problem was that building the bindings fails in a somewhat non-intuitive way. pip installs the dependencies, including Cython, but then the build process for the bindings fails. Therefore, having the dependency on the compiler documented should be helpful even though this might be a rather rare problem.