Allow mix of local and remote files in bundle.

[kyocum]

This is a feature request. Currently you must return all local files or all remote (s3) files in a bundle column. I.e., b.data = ["/some/file", "/some/otherfile"] is valid, but b.data = ["/some/file", "s3::/bucket/key/file"] is not. Note that it is currently possible to get around this limitation by returning separate bundle columns: b.data = {'local':["/local/file"], 'remote': ["s3://bucket/file"]} At a minimum we should provide a useful error message instead of silently failing, and consider supporting this mix. At the moment, one can use the Bundle object to selectively localize and delocalize file links, but this only works for pre-existing bundles. The above request would align that feature with creation as well.