merge from github

git-svn-id: https://hedgehog.fhcrc.org/bioconductor/trunk/madman/Rpacks/flowWorkspace@74115 bc3139a8-67e5-0310-9ffc-ced21a209358
Merge commit 'c09331cbcdd57aed0fe8591538fd44e7ebe338f6' into trunk
throw an error instead of returning an empty GatingSet in case of no samples found for the selected sample group. close #70
bump version