API: change nomeclature in HDFStore to use format=fixed(f) | table(t)

see #4645

Storer has been renamed to Fixed (and all docs changed)

added option: ``io.hdf.default_format`` to allow the overriding of the default format

format is now specified as: ``format=`` with a value of (``fixed(f)`` or ``table(t)``)

should be a bit more intuitve