refactoring insert_diagonal so that it can add or overwrite the diagonal weights
refactoring insert_diagonal so that it can add or overwrite the diagonal weights

dfolch/pysal,hasecbinusr/pysal,lixun910/pysal,ljwolf/pysal,spreg-git/pysal,sjsrey/pysal_core,ljwolf/pysal_core,sjsrey/pysal_core,jlaura/pysal,nathania/pysal,badjr/pysal,darribas/pysal,TaylorOshan/pysal,schmidtc/pysal,spreg-git/pysal,dfolch/pysal,jlaura/pysal,sjsrey/pysal,nathania/pysal,badjr/pysal,pedrovma/pysal,schmidtc/pysal,lixun910/pysal,lanselin/pysal,pastephens/pysal,ljwolf/pysal_core,darribas/pysal,jlaura/pysal,pastephens/pysal,pysal/pysal,chhao91/pysal,chhao91/pysal,TaylorOshan/pysal,hasecbinusr/pysal,ljwolf/pysal_core,sjsrey/pysal_core,lixun910/pysal,pastephens/pysal,hasecbinusr/pysal,ljwolf/pysal,ljwolf/pysal,TaylorOshan/pysal,weikang9009/pysal