test for "RNetCDF" in importCdf example

Add `if (require("RNetCDF"))` to `importCdf` example to avoid build ERROR on
CRAN's r-release-osx-x86_64-mavericks machine where RNetCDF is not available
don't eval importremote chunk

Do not evaluate the importremote chunk to avoid build failures on CRAN. Add also
Tan2006 data set and reference to demonstrate the `centroided=FALSE` effect.
bump version number