Simpler fast-ff interface.

Removes extra complications that seemed unnecessary and make description harder
(see https://github.com/SciTools/iris/pull/1420  -- can get simpler if we make these changes)

The API seems to contain some unnecessary confusing extras... 
  * can pass a group of filenames, but unlike ```iris.load()``` this can't combine data from multiple files into one cube
  * can specify a cube callback, but unlike ```iris.load()``` this seems a bit pointless, as it only applies it to the result cubes + can't affect the loading process.

So... this proposes to remove both those (but n.b. will need merge with #1420)

I can still see potential use for callbacks, e.g. adding/removing an attribute to/from everything, or renaming a coordinate. I appreciate it may not be as powerful as getting at a cube from each field (and the field), but I'm not convinced we should remove it.