ADD: echo classification

Adding echo classification module, including a Fortran 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 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...
MOD: minor changes
ADD: echo class wrapper

Adding f2py wrapper to the Fortran Steiner echo classification routine.
According to the SciPy docs, the distutils extensions in NumPy can be
used in setup.py to define an extension module entirely in terms of this
pyf interface file, and is the best way to distribute f2py extension
modules.
MOD: minor changes

Adding some minor changes, as well as some more documentation. The
documentation is still unfinished though...
DOC: updating documentation

Updating the documentation for the Steiner echo classification routine,
including the appropriate reference. I also checked echo_class.py
against PEP8 standards, and it looks good, except for several "blank
line contains whitespace" remarks.

One thing left to do is add the appropriate extension to the setup.py
file...
Chl Reader support
Fixed a typo
ENH: Refactor and improvements of FourDD code.

* Existing FourDD code base was refactor to improve readability and
  maintainablilty.
* Compile time parameters were exposed and can be set from Python during run
  time.
* Added support for dealiasing radars where the number of ray varies between
  sweeps.
* The functionality of the FourD dealiasing algorithm was not modified, no
  bugs where fixed, etc.  This version should give identical results as older
  version of the code.  THIS WILL BE THE LAST COMMIT WHERE THIS IS TRUE.
* A full revision history of changes made during this refactor can be found
  in the fourdd_fixes branch of GitHub user jjhelmus.
BUG: FourDD filtering based on reflectivity

When filt=1 is used during dealiasing gates where the reflectivities are
below lowdbz or above highdbz are now masked in the velocity field as well as
gates where the reflectivity is missing if rm_missing is set to 1.
BUG: wrong ineqality in FourDD filter
WIP: conda recipe
ENH: Added additional RSL Volume numbers
BUG: RSL fields not included in field mapping not dropped.

Unassigned RSL fields are not mapped to a field of name None,
rather they are skipped.
ENH: New CI Setup

* Conda recipe tested.
* Documentation is build whenever commit lands on ARM-DOE/master.
BUG: errors cause early end in build_docs.sh
Merge branch 'master' into enh_ci

Conflicts:
	pyart/correct/_fourdd_interface.c
	pyart/correct/_fourdd_interface.pyx
Merge branch 'enh_ci'
FIX: bug fix and minor enhancements

Fixing a minor bug in the Grid object as to how it handles fields with
incorrect shapes. Also adding the capability for the user to exclude fields
during a read and an add_field method to the Grid object, both of these
similar to their counterparts in the Radar object.
STY: PEP8 and docstring fix on grid.py
Merge pull request #2 from jjhelmus/pr_166

STY: PEP8 and docstring fix of grid.py

These are the style changes Jonathan requested before merging this branch into Py-ART's master branch.