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.
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