TST: split out test_pytables.py to sub-module of tests

[jreback]

(pandas) bash-3.2$ wc -l pandas/tests/io/test_pytables.py 
    5674 pandas/tests/io/test_pytables.py

should make a sub-module
pandas/tests/io/pytables

and then split out into various categories of tests, 1 file per.

[mikebarkas]

I am working on this.