Allow "apply_function" as a public function

Previously, `_apply_along_axes` was private (and misnamed w.r.t. plurality).  It makes sense to allow users to apply their own functions.  For example, there are some numpy functions (`mean`) that I wanted to apply to the data and didn't have any way to do it.

It would be good to do the same thing for the `_apply_numpy_function` tool, but that one is a little more complicated to deal with and has a deeper dependency tree.

Also, right now `apply_function` has no tests!  Tragedy and travesty!
One quick thought -- we *could* implement __getattribute__ such that cube.func_123(...) automatically dispatches to apply_numpy_function(np.func_123,...). That might be too magical