API/DOC: in HDFStore, change fmt -> format, change Storer format to Fixed format (just a name change);
         option for format is now fixed(f) or table(t)
API: add io.hdf.default_format option to enable an override to default storage formats in HDFStore
Merge pull request #4715 from jreback/hdf_format

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