Merge branch 'bugfix/cleanup-exercises' into 'master'

Cleanup exercises for dune course 2018

See merge request !44 (merged)

(cherry picked from commit 3cacd503)

f91745e4 Use less refinement as default in exercise00 a394650c Fix some typos in exercise sheet for tutorial01 8c269b83 Remove unnecessary parts from ini file b407e87a Distinguish between u_0(x) and g(x,0) 66d4b948 Update solution and remove unused ini parameter dim bd832877 Update link in tutorial04 954c8cff Small bugfixes in exercise06