robustify dimension checking in case of dynamic indexing

[paciorek]

A user noted that with RHS-only dynamically indexed variables, we produce a warning message. This is somewhat new (didn't happen in 1.1.0) and caused by some additional checking we (probably I) put in at some point. This PR relaxes that check.

Here's a working example:

code <- nimbleCode({
    for(i in 1:3) {
        y[i] <- x[z[i],1]
        z[i] ~ dcat(p[1:3])
        }
})
m <-nimbleModel(code,inits=list(z=c(3,1,2)),data=list(x=matrix(rnorm(8),8,1)))

Defining model
  [Warning] dimensions specified are larger than model specification
            for variable `x`.