Extra section 3.5 for IN4170 students

There is a bit of extra pensum through self study for the students registered in IN4170, i.e. on master level: read and understand book section '3.5 Rectifier Circuits', with an emphasis on being able to deduce and draw the qualitative I/O relationship of the rectifier circuits presented here and thus being able to apply the knowledge to reason out the I/O relationship to some similar circuits.