Fixes bug in issue #2 where false files were showing up in the listing.

* adds a `delete(row, col)` method to the `IO` interface.
* adds a `allValues(key)` method to the `IO` interface. (scaling problems noted)
* pushes responsibility of keeping track of all file names to the `RowMeta` class,
* which gets its dedicted IO instance from `ColDirectory`.
Include tests for `CachingCompositeIO` and `SplitRowIO`.

Required a change in CCIO to make it flush on `put()` so that writes could be realized in the backing `IO` without flushing.