wmoo: campaign appends to output CSV

Usually you'll want to remove weathermonitor.csv before running the
campaign.  If you forgot that but desperately need the old or the new
data, you can easily recover these manually (as they're separated by CSV
headers).

git-svn-id: https://www4.informatik.uni-erlangen.de/i4svn/danceos/trunk/devel/fail@1115 8c4709b5-6ec9-48aa-a5cd-a96041d1645a