Merge branch 'wrf' into 'master'

Since all the wrfchem-related plugins are not in the master branch yet, I sometimes have to reimplement changes that were made in 'master' in the wrf branch (just now I adapted 'wrf' to the new 'datastream' plugin type, and hope I didn't overlook anything). If we merge 'wrf' into 'master', it would allow adapting wrfchem-related plugins directly in 'master' when changes there affect them, hence this merge request. I just tried pulling 'wrf' into 'master' in a cloned respository on my machine, and it fast-forwarded, so this seems like a good point to make the merge.