Allow for processing disposable slice sources

[forman]

In addition to passing xr.Dataset instances to the zappend processor, it would be very useful to allow for passing objects that yield a slice xr.Dataset when requested. After the slice has been processed the object would be closed, giving it the chance to dispose or cleanup temporary resources.

In fact, allow for passing instances of the zappend.slicesource.SliceSource ABC as input to the processor. Users could then provide their own slice source by implementing its abstract open and close methods.