Log Message:
Update py-pytables to 2.4.0.

Major changes in 2.4:
- Improved HDF5 error logging management.
- Added support for the float16 data type.
- Leaf nodes now have attributes for retrieving the size of data in memory
  and on disk.
- Configurable maximum number of threads for Blosc and Numexpr.
- ndim (read-only) attribute added to Leaf, Atom and Col objects.
- Added read support for variable length string attributes.

Full changelog for 2.4.0, see:

  http://pytables.github.io/release-notes/RELEASE_NOTES_v2.4.x.html

Major changes in 2.3:

- Integrated functionality from PyTablesPro (re-licensed under a BSD license).
- OPSI is a powerful and innovative indexing engine allowing PyTables
  to perform fast queries on arbitrarily large tables.
- A fine-tuned LRU cache for both metadata (nodes) and regular data.

Full changelog for 2.3.x, see:

  http://pytables.github.io/release-notes/RELEASE_NOTES_v2.3.x.html