Merge pull request #4430 from jreback/testing

TST: better assertion messages on test failures (GH4397)
API: GH4409 HDFStore addes an is_open property to indicate if the underlying file handle is_open;
    a closed store will now report 'CLOSED' when viewing the store (rather than raising an error)

API: HDFStore, removed the _quiet attribute, replace by a ``DuplicateWarning`` if retrieving
     duplicate rows from a table (GH4367)

TST: change warnings filter to assert_produces_warning in test_pytables.py

API: removed the warn argument from open. Instead a PossibleDataLoss exception will
     be raised if you try use mode='w' with an OPEN file handle (GH4367)

API: semantics of close in HDFStore now will close that instance of HDFStore
     the final file close is ref counted (by PyTables) w.r.t all of the open handles

ENH: raise ClosedFileError if an operation is attempted on a closed file