minor modifications to the hierarchical kern 
minor modifications to the hierarchical kern 

TianpeiLuke/GPmat,TianpeiLuke/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,SheffieldML/GPmat