Mention further dependencies in installation guide

Current situation

At the moment, the homepage lists the following mandatory preprequisites

• C++17 compiler (GCC 7 or newer, Clang 5 or newer)
• CMake (DuMux 3.3-git requires at least CMake 3.13; DuMux <= 3.2 requires at least CMake 3.1)
• pkg-config

and the following optional preprequisites

• MPI (either OpenMPI, lam, or mpich)
• ParaView (to visualize the results)
• a browser (to access the GitLab instance and README files)

The first step of the installation guide executes the installation script via

python3 installdumux.py

which also requires Python 3, wget and git (not sure which minimum versions).

While one would need wget and git only for the installation one could argue that Python is also run dependency for DuMuX as several Python scripts are included in DuMuX.

Expected situation

All optional and mandatory dependencies are mentioned in the documentation.

Proposed solution

I would recommend mentioning all dependencies explicitly as it is not guaranteed that the mentioned dependencies are always preinstalled by default.