switched from tables.Array to tables.EArray

Use tables.EArray instead of tables.Array, and add data  incrementally,
to cut down on large memory overhead. Ran into problems with concurrent
read from the same tables.File object
(https://github.com/PyTables/PyTables/issues/292); giving each worker
its own File resolved the issue. Nervous about the update functions
called by each worker, but so far no issues…