Add an open method to each source module.

[manthey]

This also adds a canRead method to each source module and an open method to the primary module.

Before, to open a file with a specific file source, you needed to do something like large_image_source_tiff.TiffFileTileSource(path), where the name of the tile source's class had to be used. Now, you can do large_image_source_tiff.open(path), which maps to the same thing. The old way still works. Similarly, canRead required something like large_image_source_tiff.TiffFileTileSource.canRead(path) and now large_image_source_tiff.canRead(path) works as well.

open is now also a synonym for getTileSource, so generically, large_image.getTileSource(path) can be written as large_image.open(path).

There is no requirement that tile source modules expose the open and canRead methods directly; it is more a convenience for developers.