Draft: Add example of color in x-y plots

[kbarnhart]

Related to clawpack/visclaw#321

[rjleveque]

Thanks @kbarnhart, nice example.

It works fine, but does throw a warning about divide by zeros (later overwritten by nan's). If you want to avoid this, you could use:

    def r_surface_and_vel(current_data):
        from numpy import sqrt, nan, divide, zeros
        x = current_data.x
        y = current_data.y
        h = current_data.q[0,:,:]
        dry_tol = 1e-3
        u = divide(current_data.q[1,:,:], h,
                   where=h>dry_tol, out=zeros(h.shape))
        v = divide(current_data.q[2,:,:], h,
                   where=h>dry_tol, out=zeros(h.shape))
        eta = current_data.q[3,:,:]
        vel = sqrt(u**2 + v**2)
        vel[h<dry_tol] = nan
        eta[h<dry_tol] = nan
        r = sqrt(x**2 + y**2)
        return r,eta,vel

This sets u,v to 0 where h <= dry_tol, which is generally what's wanted instead of inf.
Or you could use out=nan*zeros(h.shape) in divide() to set u,v to nan more directly whereever h <= dry_tol.

[kbarnhart]

@rjleveque - I've revised to incorporate your suggestions. Thanks!

[rjleveque]

Thanks again!