ENH: 4DD routine

Adding the capability to use a previous velocity volume as an initial condition
to the dealiasing routine. Note that this enhancement was how the original 4DD
algorithm was intended to be implemented, as it should provide better results.
This commit attempts to address some of the issues raised in issue #119.
ADD: Cython generated _fourdd_interface.c

Adding the generated C file from the Cython build since Cython isn't
a build dependency of Py-ART.
MOD: _fourdd_h.pxd

Attempting to expose some of the defined constants in the header file
FourDD.h so that the user knows what the values of some of these
constants are at compile-time. As with discussion with @jjhelmus, these
constants cannot actually be changed at run-time; they must be changed
before compiling.
MAINT: refactor of 4DD improvements

This improves on the enhancement to Py-ART 4DD wrapper which allows for
dealiasing using previously unfolded radar volumes.  This makes it possible
to dealiasing using either an unfolded volume and/or sounding data.

In addition the follow minor fixes were complete:

* Unit test for this new functionality were added.
* Python and Cython files we corrected to follow the PEP8 style guide.
* The plot_dealias.py example was updated to reflect the new syntax.
ENH: beam width parameters when reading RSL files.
ENH: beam width parameters when reading Sigmet files
ENH: beam width parameters when reading MDV files.
BUG: writing cfradial file without instrument_parameters

Writing a Cf/Radial file from a radar object in which the instrument_parameters
attribute is None no longer errors out.
MAINT: Error message when reading nexrad msg1 files
Merge pull request #151 from jjhelmus/master

ENH: beam width parameter included when reading files
Merge pull request #1 from jjhelmus/pr_132

MAINT: refactor of 4DD improvements
Merge pull request #132 from kirknorth/iss119

ENH: Dealias using 4DD using previous radar volumes.
BUG: RadarDisplay colorbar

If the user was using the RadarDisplay class in a loop and making many
plots all with individual colorbars, then the last plot would receive
all the colorbars since the axis was not being specified properly in the
call to plot_colorbar.
Merge pull request #154 from kirknorth/radar-display-cbar

BUG: RadarDisplay colorbar
BUG: One-byte data types in Sigmetfiles

One-byte data types present in Sigmet files are read correctly.
Of note is the VEL field which is now returned in units of meters per second
as apposed to percent of nyquist velocity.
Merge pull request #155 from jjhelmus/master

BUG: One-byte data types in Sigmetfiles
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