Change the default estimation method for gam in geom_smooth()

[clauswilke]

As recommended by the maintainer of the mgcv package, Simon Wood, it might be better to set method = "REML" in the gam() call for geom_smooth(). I quote:

REML smoothing parameter estimation is less prone to occasional under-smoothing than GCV (e.g. gam(..., method="REML"))

[clauswilke]

One more comment from Simon Wood:

Theoretically REML will undersmooth relative to GCV in the large sample limit and have slightly worse MSE performance, but for me the finite sample performance trumps this: Reiss and Ogden, 2009, JRSSB give some theory for why.

The Reiss and Ogden paper is available here: http://www.jstor.org/stable/40247585

gam(..., method = "REML") will work for mgcv versions >=1.5-0, March 2009.

[hadley]

This should be a straightforward tweak to the existing code, and just needs a news bullet. It is likely to create small changes in some existing plots, but I don't think the differences should be too serious (although it might cause visual tests to fail)

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