MAINT: Refactor of Steiner echo classifier.

* Merged with ARM-DOE master branch, this will not cause issues once incorporated into PR #135 as these should be ignored.
* Created a setup.py file which conditionally compiles the Fortran 90 module.
* A pyart.retrieve namespace was added to which the steiner_conv_strat
  function was added.
* Function added to the documentation.
* Unit test added, new grid object added to testing module.
* Bug with masked array fixed  in steiner_conv_strat function.
* Extension module renamed to _echo_steiner.