Add KKSS pruning for third order

According to Den Haan/De Wind (2012), the pruning currently implemented
at order 3 is not the KKSS pruning as the third order term gyy12 uses
the second order yhat2. This commit adds both the traditional KKSS
pruning for order=3. The KKSS-option and the standard
pruning currently implemented at order 3 should potentially be
consolidated.