Handle multi-file data formats

[apdavison]

Many ephys file formats handled by Neo spread their data over multiple files in a single directory, typically with the same filename root but different extensions.

Neo-Viewer cannot currently handle such formats.

There are three scenarios for the Neo IO module:

1. expects a single filename, and all data are in that file (e.g. PlexonIO, most other IOs)
2. expects a single filename, data are contained in that file and other files with the same root but different extensions (e.g. BlackRockIO, AsciiSignalIO)
3. expects a directory name (e.g. PhyIO)

For scenarios 1 and 2, providing a single URL is fine. For scenario 3, a single URL representing a directory may be sufficient (where the names of the files within the directory are fixed).

For scenarios 2 and 3 we should also support providing the URL of a zip or tar archive.

[apdavison]

zip file support added in 698db2a. tar file support still to do.