ENH: Gridding without reflectivity filter.

Introduces a `refl_filter_flag` parameter to to **map_to_grid** function
which controls filtering based on the reflectivity field.

closes #98
OPT: optimized sobel filter in phase_proc.py

Vectorized sobel filter used in calculating KDP and replace
with call to scipy.ndimage version of convolve as suggested
by kmuehlbauer.

Closes #95
ENH: Keep original velcocities when dealias fails.

Introduces the `keep_original` parameter to the dealias_fourdd function
which when given a value of True will replaces gates where dealiasing fails
with the original doppler velocities.

This was enhancement was suggested by Kirk North

Closes #97
DOC: fixed docuementation of ax_radius
ENH: extract_sweeps Radar method

The extract_sweeps method can be used to create a Radar object which
contains one or more sweeps from a larger radar object.

Closes #99