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)

The variety of load, load_cube, load_cubes is confusing enough. The fact they all take one or more filenames is one bit of consistency we might want to hold on to. Putting that aside and taking this function in isolation, there is the possibility that I have multiple fieldsfiles I want to load everything from (quickly) so I can take a look. This change means I have to write a loop and the combine the results into a single list. I'm :-1: on this, but I can see where you are coming from.