Initial Changes to include rigid loading option

bucklingVAT.m

- switch added for two different types of loading case

- Add buckling load as out ... if different loading cases lambda
(eigenvalue) means different things, for rigid loading it is the strain
for dead it is the load. To take account for this the output for
buckling load is rename to Pc

stiffnessVAT.m

- BUG: th = sum(model.plyths) is incorrect chaned to th = sum(plyths);

- mass is now calculated in stiffnessVAT, mass in each element is
4*detJ*th*model.material.density
Back track... to account for my cock up

I have backtracked to last stable version sent by Jon on 22/07/2014

then implemented a number of changes to stiffnessVATupdate