Fix/rendering markdown equations

!Closes dumux#1303 (moved)

Fixes rendering issues for some of the slides. Some minor formation hiccups(slightly wrong indented lines, slightly cut of equations at the top) or not properly written Dumux's are ignored. Slides previously already in a good state:

• coupled.md
• dunemodule.md
• grid.md
• multidomain.md
• params.md
• properties.md
• python.md

Improved slides:

• biomin.md
• fractures.md
• intro.md
• materialsystem.md
• problem.md

Slides still needing improvement:

• biomin.md
• intro.md
• model.md