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
