Allow XML specification of auto-optimization for Skygrid

git-svn-id: http://beast-mcmc.googlecode.com/svn@6522 ca793f91-a31e-0410-b540-2769d408b6a1
Change default for Skygrid precision sampling

git-svn-id: http://beast-mcmc.googlecode.com/svn@6523 ca793f91-a31e-0410-b540-2769d408b6a1
Change default skygrid behaviour; no simultaneous sample of precision and field

git-svn-id: http://beast-mcmc.googlecode.com/svn@6524 ca793f91-a31e-0410-b540-2769d408b6a1