Add deprecate decorator

There have been a few places I've seen where we've wanted to remove/deprecate a function (see, for example, the __call__ operator on the ComponentAccumulator and AthConfigFlags classes). Having this decorator would make this much easier...

As-is it requires a reason for the deprecation to be given, in which we could suggest an alternative function to use.

I imagine that folk like @fwinkl, @wlampl or @dguest may have opinions (and indeed many others besides...)