codyn fails to import 'stats' package methods

[mbjones]

Newer versions of R require that 'stats' methods be explicitly imported rather than being assumed as available. Add 'importFrom' throughout the R code, and rebuild the NAMESPACE to support those.

In the future, ensure there is an ROxygen import statement for every library in use.

[mbjones]

@laurenmh and @aammd -- note that for future additions we need to properly import functions -- you need lines like this for every external function that you use:

#' @importFrom stats aggregate as.formula sd

Alternatively, if you prefer to import the whole package, you can use:

#' @import stats