Coefficient storage

[lindsayad]

It seems like coefficients created [Coefficients] syntax and those created behind the scenes associated with [Materials] are stored in separate data members. Might it make sense to store them in one data member and that way user code could retrieve coefficients without thinking about whether these coefficients were created through different syntaxes?

[cmacmackin]

We are getting rid of the concept of explicitly created coefficients in #53, so that issue should go away.