Draft: Small stepgrid change

[kbarnhart]

Another change based on D-Claw development: In D-Claw we have one variable in q that should not be set to zero when q(1)<drytol.

This suggestion is intended to retain geoclaw functionality, but not require a D-Claw specific routine that differs only by this line.

@rjleveque @mandli @dlgeorge

[mandli]

This is probably true for any field that is momentum related I suppose. Most of the other fields, unless they somehow have velocity issues, would not necessarily want to zero it out either. In the multilayer code we do the following:

      forall(i=1:mitot, j=1:mjtot, k=1:num_layers,
     &       q(3*(k-1)+1,i,j) / rho(k) < dry_tolerance(k))
        q(3*(k-1)+1,i,j) = max(q(3*(k-1)+1,i,j), 0.d0)
        q(3*(k-1)+2,i,j) = 0.d0
        q(3*(k-1)+3,i,j) = 0.d0
      end forall

Given the momentum equations are interspersed with the other fields, this also required another stepgrid.f.

In any case, modulo the bouss code, I think being explicit about this is probably fine. The alternative would be to call something that would do only this custom bit. I suppose this might be a recurring task that may be easier to implement that way, but that may be too complex a solution for this.