Add quadrature-based Prism6::volume() implementation.
Add quadrature-based Prism6::volume() implementation.

This turned out to be slightly faster (about 20%) than the existing
geometric volume computation, which was itself likely already much
faster than the base class Elem::volume() method for this element,
which I did not bother to measure.

90jrong/libmesh,capitalaslash/libmesh,90jrong/libmesh,aeslaughter/libmesh,libMesh/libmesh,dschwen/libmesh,giorgiobornia/libmesh,giorgiobornia/libmesh,pbauman/libmesh,balborian/libmesh,balborian/libmesh,balborian/libmesh,coreymbryant/libmesh,vikramvgarg/libmesh,BalticPinguin/libmesh,friedmud/libmesh,friedmud/libmesh,svallaghe/libmesh,roystgnr/libmesh,balborian/libmesh,svallaghe/libmesh,capitalaslash/libmesh,pbauman/libmesh,svallaghe/libmesh,90jrong/libmesh,capitalaslash/libmesh,pbauman/libmesh,dschwen/libmesh,giorgiobornia/libmesh,BalticPinguin/libmesh,roystgnr/libmesh,coreymbryant/libmesh,benkirk/libmesh,giorgiobornia/libmesh,vikramvgarg/libmesh,roystgnr/libmesh,benkirk/libmesh,capitalaslash/libmesh,capitalaslash/libmesh,svallaghe/libmesh,jwpeterson/libmesh,dschwen/libmesh,jwpeterson/libmesh,vikramvgarg/libmesh,balborian/libmesh,vikramvgarg/libmesh,libMesh/libmesh,vikramvgarg/libmesh,vikramvgarg/libmesh,benkirk/libmesh,coreymbryant/libmesh,dschwen/libmesh,hrittich/libmesh,capitalaslash/libmesh,BalticPinguin/libmesh,dschwen/libmesh,pbauman/libmesh,hrittich/libmesh,aeslaughter/libmesh,dschwen/libmesh,giorgiobornia/libmesh,hrittich/libmesh,BalticPinguin/libmesh,libMesh/libmesh,svallaghe/libmesh,pbauman/libmesh,hrittich/libmesh,roystgnr/libmesh,svallaghe/libmesh,pbauman/libmesh,capitalaslash/libmesh,balborian/libmesh,jwpeterson/libmesh,hrittich/libmesh,svallaghe/libmesh,vikramvgarg/libmesh,balborian/libmesh,benkirk/libmesh,friedmud/libmesh,libMesh/libmesh,jwpeterson/libmesh,coreymbryant/libmesh,coreymbryant/libmesh,friedmud/libmesh,BalticPinguin/libmesh,aeslaughter/libmesh,libMesh/libmesh,svallaghe/libmesh,90jrong/libmesh,friedmud/libmesh,hrittich/libmesh,aeslaughter/libmesh,benkirk/libmesh,BalticPinguin/libmesh,balborian/libmesh,libMesh/libmesh,jwpeterson/libmesh,jwpeterson/libmesh,giorgiobornia/libmesh,jwpeterson/libmesh,dschwen/libmesh,coreymbryant/libmesh,90jrong/libmesh,90jrong/libmesh,libMesh/libmesh,BalticPinguin/libmesh,benkirk/libmesh,hrittich/libmesh,giorgiobornia/libmesh,giorgiobornia/libmesh,hrittich/libmesh,jwpeterson/libmesh,dschwen/libmesh,giorgiobornia/libmesh,friedmud/libmesh,friedmud/libmesh,balborian/libmesh,aeslaughter/libmesh,aeslaughter/libmesh,svallaghe/libmesh,aeslaughter/libmesh,libMesh/libmesh,vikramvgarg/libmesh,friedmud/libmesh,90jrong/libmesh,roystgnr/libmesh,roystgnr/libmesh,aeslaughter/libmesh,90jrong/libmesh,friedmud/libmesh,BalticPinguin/libmesh,benkirk/libmesh,roystgnr/libmesh,balborian/libmesh,90jrong/libmesh,roystgnr/libmesh,hrittich/libmesh,vikramvgarg/libmesh,aeslaughter/libmesh,pbauman/libmesh,benkirk/libmesh,pbauman/libmesh,capitalaslash/libmesh,coreymbryant/libmesh,pbauman/libmesh,benkirk/libmesh,coreymbryant/libmesh