Add learning curves and edit notebook `04 - Cross Validation`
Add learning curves and edit notebook `04 - Cross Validation`

Edit notebook `04 - Cross Validation`. Remove exercises from seminar
and provide examples for users to mimic. Add learning curves.

Adress #46

awhite40/pymks,davidbrough1/pymks,davidbrough1/pymks,fredhohman/pymks,XinyiGong/pymks