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.
FIX: Ensuring proj matches basemap
ENH: Added point plotting on maps
FIX: A couple of bugs with the first commit
ENH: range line segment plotting and range rings on radar maps
FIX: Minor typos and bugs
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.
Added an example for basemap plotting
BUG: Fixed label offset
ENH: Can now read Sigmet files with some additional data types
Merge pull request #143 from jjhelmus/sigmet_field_72_74

ENH: Can now read Sigmet files with some additional data types
MAINT: Clean up for enhancements to RadarMapDisplay

* Added documentation to methods.
* Fixed example of RadarMapDisplay.
* Fixed mismatch in projections.
* Added unit tests.
Merge pull request #134 from scollis/fix_map_background

Fix map background
Merge pull request #145 from jjhelmus/pr_134

MAINT: Clean up for enhancements to RadarMapDisplay
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.
DOC: added links to standard in write_cfradial
Merge pull request #147 from jjhelmus/master

DOC: added links to standards in write_cfradial docstring
BUG: read_sigmet has wrong fixed_angle data type

Using the read_sigmet routine followed by write_cfradial now results in
the fixed_angle variable having a float type as apposed to a double.
Merge pull request #149 from jjhelmus/master

BUG: read_sigmet has wrong fixed_angle data type