Calculates the diagonal of the covariance function for the OGP.
covf = ogpcovdiag(x1);
[covf] = ogpcovdiag(net, x1) takes an OGP data structure
net together with a data matrix x1 and
computes the diagonal elements of the covariance function.
If the output is not one-dimensional, then the returned matrix is
[dim(x1),nout,nout] dimensional.
ogp, ogpinit, ogpcovarf, ogpcovarp
Copyright (c) Lehel Csató (2001-2004)