Tahoe Code SVN development

density here and in the solid material have different meaning and can't
be the same.

added accessor the species density

check that density for the mixture species in the associated element class
is the same as the density here.

added method to return the material as a mixture with errorchecking, etc.

corrected modulus for species concentration solved in the current configuration

parameter initialization changed to match changes to fung-type potentials

Potentials now depend r^2 instead of r

always keep the associated solid element group synchronized when running through
element calculations with Top-Next

need to reset the format of the element stiffness matrix based on changes to
the concentration type

changes to support concentrations with respect to the current configuration. The tangent
modulus still needs to be implemented, so use the finite difference approximation for now.