New inference rule: Case distinction based on implication
proof tree visualization improved for non-branching tree
Merge branch 'prove'
Extension domains for computation and proving

Not yet implemented/working: condition when to switch to base domain
prove/compute settings display without absolute file paths
Proof simplification

step elimination and formula elimination.
Works in some examples. Not too heavily tested.
proof tree visualization improved for non-branching tree

AspectRatio is set again, but we check whether we have a non-branching tree or not.
Display empty set
checking conditions in rewriting debugged, solving by matching debugged
Minor proof text revisions