Rework Ubuntu Installation Instructions

[Bengt]

• Be explicit about Ubuntu version numbers
• Clone repository first to keep the venv local
• Be explicit about Python version numbers
• Prefer using apt instead of apt-get
• Add --yes flag to apt install subcommand
• Use a virtual environment to not pollute the system
• Install python packages in one command so that pip can resolve their dependencies properly
• Use python executables from virtual environment for running analyse.py script