Matt McCormick authored Oct 12, 2016 and Brad King committed Oct 18, 2016

Add support for projects to have `Data.txt.md5` *and* `Data.txt.sha512`
where the content links hold hashes for the same file.  Check all
`ExternalData_URL_TEMPLATES` entries in order for all available hashes.
The data acquisition is considered a failure if none of the available
URL resources has any of the given hashes.  This makes it possible to
have multiple data server resources where all servers do not support all
hashing algorithms.