Fix for Rossby wave source examples

I noticed a small error in your calculation for the Rossby wave source whereby this,
`S = -eta * div - uchi * etax + vchi * etay`
should actually be
`S = -eta * div - (uchi * etax + vchi * etay)`

When viewing plots of `S` the error isn't really noticeable by eye. It was only when I broke down the calculation into the `S1 = -eta * div` and `S2 = - (uchi * etax + vchi * etay)` components as [Shimizu and de Albuquerque (2011)](http://link.springer.com/article/10.1007%2Fs00382-010-0841-z) does that I noticed the error in the `S2` component (i.e. I was trying to reproduce Figure 6 of that paper using your windspharm example).