Symbolic tracing in staggered grid can return Nans

[cmhyett]

The way tracing was implemented in staggered_discretize.jl zeros were padded to dynamic variables to ensure we didn't get two copies of the update when performing the split step integration. Unfortunately this is not at all the right way to do this. It is not correct if we have inhomogeneous terms and can return Inf if we divide by the variable.