Function to download collection data (grouped texts) from github. Overwrites (!) the current collections.
installCollectionData(dataDirectory = getOption("qd.datadir"),
branchOrCommit = "master", repository = "metadata",
baseUrl = "https://github.com/quadrama/")
The data directory in which collection and data files are stored
The git branch, commit id, or tag that we want to download
The repository
The github user (or group)
NULL