ADD: echo classification

Adding echo classification module, including a <b>Fortran</b> routine for the method of Steiner et al. (1995). The echo classification module should be general and host any number of methods (e.g. Steiner method, Biggerstaff method, etc). This commit does not (yet) include the necessary object and/or pyf files required to actually run the code, as I figured @jjhelmus would be best at handling this. I assume some modification to the setup.py file in pyart.retrieve is in order...