Add algorithm file support to evaluator

This pull request adds algorithm support to the evaluator (#406).

This allows configuration like this:

~~~groovy
dumpGraph {
    algorithm 'algoSpec.groovy'
}

dumpGraph {
    algorithm 'algo2.groovy', name: 'Algorithm2'
}
~~~

The individual algorithm files are only LensKit algorithm configs, as loaded by `lenskit-groovy`. They cannot configure other aspects of the algorithm instance.