Add exercise to train simplification of if-else-expressions

[valterbergstrand]

Title explains but an example would be something in the line of:

def isTrue(): Boolean = 
  if boolsktVärde == true then true
  else false

can be simplified to

def isTrue(): Boolean =
  boolsktVärde

[bjornregnell]

I have added this slide:

[bjornregnell]

TODO: Consider if also add an exercise in week 01 pr week 02. Check what's already there first...