Adding second derivatives for Hex20.
Adding second derivatives for Hex20.

Maple script used for computing the derivatives:
https://gist.github.com/jwpeterson/7555673

giorgiobornia/libmesh,svallaghe/libmesh,vikramvgarg/libmesh,coreymbryant/libmesh,vikramvgarg/libmesh,balborian/libmesh,BalticPinguin/libmesh,dknez/libmesh,vikramvgarg/libmesh,vikramvgarg/libmesh,vikramvgarg/libmesh,libMesh/libmesh,coreymbryant/libmesh,jiangwen84/libmesh,roystgnr/libmesh,svallaghe/libmesh,jiangwen84/libmesh,karpeev/libmesh,benkirk/libmesh,benkirk/libmesh,cahaynes/libmesh,karpeev/libmesh,benkirk/libmesh,libMesh/libmesh,Mbewu/libmesh,jiangwen84/libmesh,90jrong/libmesh,roystgnr/libmesh,svallaghe/libmesh,BalticPinguin/libmesh,jwpeterson/libmesh,capitalaslash/libmesh,aeslaughter/libmesh,jwpeterson/libmesh,pbauman/libmesh,libMesh/libmesh,dmcdougall/libmesh,karpeev/libmesh,capitalaslash/libmesh,dschwen/libmesh,roystgnr/libmesh,vikramvgarg/libmesh,aeslaughter/libmesh,karpeev/libmesh,pbauman/libmesh,salazardetroya/libmesh,permcody/libmesh,aeslaughter/libmesh,Mbewu/libmesh,hrittich/libmesh,libMesh/libmesh,jwpeterson/libmesh,balborian/libmesh,balborian/libmesh,salazardetroya/libmesh,cahaynes/libmesh,dschwen/libmesh,permcody/libmesh,BalticPinguin/libmesh,hrittich/libmesh,90jrong/libmesh,aeslaughter/libmesh,permcody/libmesh,svallaghe/libmesh,dknez/libmesh,hrittich/libmesh,friedmud/libmesh,roystgnr/libmesh,libMesh/libmesh,vikramvgarg/libmesh,vikramvgarg/libmesh,salazardetroya/libmesh,karpeev/libmesh,benkirk/libmesh,balborian/libmesh,giorgiobornia/libmesh,90jrong/libmesh,benkirk/libmesh,friedmud/libmesh,friedmud/libmesh,svallaghe/libmesh,pbauman/libmesh,hrittich/libmesh,dmcdougall/libmesh,jiangwen84/libmesh,Mbewu/libmesh,pbauman/libmesh,dschwen/libmesh,dknez/libmesh,roystgnr/libmesh,giorgiobornia/libmesh,cahaynes/libmesh,90jrong/libmesh,libMesh/libmesh,capitalaslash/libmesh,karpeev/libmesh,pbauman/libmesh,jwpeterson/libmesh,Mbewu/libmesh,friedmud/libmesh,hrittich/libmesh,hrittich/libmesh,BalticPinguin/libmesh,aeslaughter/libmesh,Mbewu/libmesh,giorgiobornia/libmesh,capitalaslash/libmesh,jiangwen84/libmesh,dmcdougall/libmesh,coreymbryant/libmesh,capitalaslash/libmesh,balborian/libmesh,BalticPinguin/libmesh,giorgiobornia/libmesh,friedmud/libmesh,dknez/libmesh,capitalaslash/libmesh,benkirk/libmesh,friedmud/libmesh,capitalaslash/libmesh,coreymbryant/libmesh,friedmud/libmesh,benkirk/libmesh,dmcdougall/libmesh,permcody/libmesh,giorgiobornia/libmesh,libMesh/libmesh,dmcdougall/libmesh,cahaynes/libmesh,coreymbryant/libmesh,karpeev/libmesh,salazardetroya/libmesh,roystgnr/libmesh,dschwen/libmesh,90jrong/libmesh,permcody/libmesh,giorgiobornia/libmesh,cahaynes/libmesh,aeslaughter/libmesh,friedmud/libmesh,svallaghe/libmesh,capitalaslash/libmesh,jiangwen84/libmesh,pbauman/libmesh,90jrong/libmesh,balborian/libmesh,dschwen/libmesh,dschwen/libmesh,balborian/libmesh,permcody/libmesh,cahaynes/libmesh,salazardetroya/libmesh,svallaghe/libmesh,Mbewu/libmesh,svallaghe/libmesh,BalticPinguin/libmesh,jiangwen84/libmesh,90jrong/libmesh,pbauman/libmesh,jiangwen84/libmesh,karpeev/libmesh,balborian/libmesh,dmcdougall/libmesh,aeslaughter/libmesh,balborian/libmesh,giorgiobornia/libmesh,svallaghe/libmesh,aeslaughter/libmesh,BalticPinguin/libmesh,libMesh/libmesh,permcody/libmesh,jwpeterson/libmesh,Mbewu/libmesh,hrittich/libmesh,pbauman/libmesh,coreymbryant/libmesh,hrittich/libmesh,giorgiobornia/libmesh,coreymbryant/libmesh,permcody/libmesh,benkirk/libmesh,90jrong/libmesh,salazardetroya/libmesh,Mbewu/libmesh,90jrong/libmesh,hrittich/libmesh,jwpeterson/libmesh,roystgnr/libmesh,aeslaughter/libmesh,pbauman/libmesh,Mbewu/libmesh,benkirk/libmesh,balborian/libmesh,cahaynes/libmesh,dknez/libmesh,dmcdougall/libmesh,friedmud/libmesh,coreymbryant/libmesh,vikramvgarg/libmesh,dschwen/libmesh,jwpeterson/libmesh,dschwen/libmesh,roystgnr/libmesh,cahaynes/libmesh,salazardetroya/libmesh,dknez/libmesh,jwpeterson/libmesh,dknez/libmesh,dknez/libmesh,dmcdougall/libmesh,BalticPinguin/libmesh,salazardetroya/libmesh