Fem system elem_solution_rate API changes

This is now passing my tests in libMesh and with a suitably changed GRINS.

@pbauman and @vikramvgarg should take a look at this and verify that I'm not doing anything they think is nuts.

This is a *backwards-incompatible* change for any DifferentiableSystem users who are overriding a mass_residual() method or doing anything similarly sophisticated.  We should *not* merge this into 0.9.4; it's slated for 1.0.0.