tweak gpKernelPlot

https://github.com/probml/pyprobml/blob/master/notebooks/book2/18/gpKernelPlot.ipynb

Change ordering to
```
rbf(0.1), rbf(1), rbf(5)
mat12, 32, 52
periodic, cosine,  RQ
poly1, poly2, noise
```

Put more space between rows

Also save as individual figures/files in addition to a 4x3 montage.
