Updated to latest numpydoc source
DOC: Documented RSL Cython wrapper
BUG: fixed rsl field name error
ENH: Refactored Cython wrappers around RSL and FourDD

* _rsl_interface.pxd split into _rsl_h.pxd and _fourdd_h.pxd
* _fourdd_interface.pyx split from _rsl_interface.pyx
* Moved fourdd related code to pyart/correct directory
* _rsl_interface now checks for NULL during Volume, Sweep and Ray creation.
* _rsl_interface now deallocated memory when objects are deleted.
* create_volume in _rsl_interface now sets the f and invf parameters based on a
  volume number
Merge pull request #46 from jjhelmus/master

Updated and documented Cython wrapped around RSL and FourDD algoriths