Retrieve mapping scheme notation convenience.

This PR provides an API convenience to ```fuseki.retrieve_mappings``` that allows the *source* and *target* scheme to be simple string notations i.e. rather than forcing the user to specify a fully qualified metOcean URI such as ```"<http://www.metarelate.net/metOcean/format/um>"'``` they may simply specify the string ```"um"```.