allow recursively adding directories as input/output

[lukemarsden]

Currently to add all files in a directory as inputs or outputs, you have to do something like

for file in os.listdir("model/1"):
    ds.output("model/1/" + file)

But even that doesn't work, because it doesn't recurse more than one layer deep.

Ideally we'd be able to just say ds.output("model") and the library would deal with doing the necessary os.walk to recursively add all the files in the directory (if the string passed to ds.output is a directory). Same for ds.input.

[lukemarsden]

Please also rename ds.summary to ds.metric (leave ds.summary as an alias for backward compatibility). Thanks!