improve biomin exercise

• remove urea.hh from exercisebiomin/components as it is now in dumux-stable already.
• fix README.md: there is some weird issue with the subscripts and the following all being formatted as subscript
• fix README.md: correct make command make exercise_biomin instead of make exercisebiomin
• fix README.md: make the difference between reaction rates and source and sink terms more clear. There was some confusion during the course.
• consistent names for parameters in chemistry and the README.md to minimize confusion

see !219 (merged)