Fix warnings in ggs_autocorrelation

The problem came from the use of 1:nLags which actually had several
variables. What Xavier struggled with had no solution back then,
but is now fixed in https://github.com/hadley/plyr/issues/3 with the
newly introduced function `here`.
Compare lags to number of iterations

Otherwise, an error was generated: now it's only a warning, we
plot as far as we can.
Merge branch 'here' of https://github.com/jucor/ggmcmc into jucor-nlags
It fixes a warning from ggs_autocorrelation() and rewrites some old code that a new version of plyr has turned easier.