Converting a tree into a librdf model and then serializing is slow, requires a lot of memory, and is not feasible for large trees. Refactoring CDAOIO.Writer to write triples directly to file instead of using librdf. This also makes the API more similar to other Bio.Phylo modules.
Default tree URI is blank, but still create a tree object.
Better tree URIs; avoid duplicating the tree ID.
Fixing some URI handling issues to make loading via librdf and isql identical.
Don't add a slash to tree_uri if it's blank