Allowed no time averaging as a column heading in NAME files

We recently added functionality to Iris (#835) that parses the "Time Av or Int" entry in NAME files to create a cell method. It turns out there are files with the string "No time averaging" in them that causes the new code to raise an exception. This code adds support for this string and also relaxes the load process so it raises a warning (and does not create a cell method) for unknown entries, rather than raising an exception.