Sophisticated data retrieval from repos

Version 7.6 introduced more sophisticated data retrieval from GitHub:

• not only zips attached to releases, but any subfolder from any commit

• The TF-API makes use of the new capabilities through the parameter checkout with values

• '' (default): latest release or latest commit

• latest: latest release

• hot : latest commit

• xxx: specific version tag

• fff : specific commit hash

• local: the available local copy in the text-fabric-data folder

• clone: the available copy in a local repo clone