ENH: Add column example to docs (#1263)

* add column example to docs

* condense some lines

examples/retrieve/README.txt

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+.. _retrieve_examples:
+
+Retrieval Examples
+------------------
+
+Retrievals from various radars, such as additional fields or subsets of the data.

examples/retrieve/plot_column_subset.py

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+"""
+====================================
+Extract a radar column above a point
+====================================
+
+Given a radar and a point, extract the column of radar data values above
+a point
+"""
+
+# Author: Maxwell Grover ([email protected])
+# License: BSD 3 clause
+
+import pyart
+import numpy as np
+import matplotlib.pyplot as plt
+import cartopy.crs as ccrs
+from pyart.testing import get_test_data
+
+# Read in some test data
+filename = get_test_data('swx_20120520_0641.nc')
+radar = pyart.io.read(filename)
+
+######################################
+# **Plot the first sweep and our desired point**
+#
+# Let's visualize our radar data from a single sweep, and plot
+# the location of our desired point on a map.
+# This will provide some context as to where we are extracting our
+# column of values.
+
+site_lon = -97.73 # longitude in degrees
+site_lat = 36.41 # latitdue in degrees
+
+# Setup the RadarMapDisplay and add our projection
+display = pyart.graph.RadarMapDisplay(radar)
+ax = plt.subplot(111, projection=ccrs.PlateCarree())
+
+# Visualize the reflectivity field, using the lowest sweep with
+# latitude and longitude lines
+display.plot_ppi_map('reflectivity_horizontal',
+                     0, ax=ax,vmin=-32, vmax=64.,
+                     lon_lines=np.arange(-98, -97, .2),
+                     lat_lines=np.arange(36, 37, .2))
+
+# Plot our site location on top of the radar image
+ax.scatter(site_lon,
+           site_lat,
+           color='black');
+
+######################################
+# Now that we have our point defined, and our radar object, we can use the following 
+# utility function in Py-ART to subset a column
+ds = pyart.util.columnsect.get_field_location(radar, site_lat, site_lon)
+
+######################################
+# This function returns an xarray dataset, with all of our data fields!
+print(ds)
+
+######################################
+# **Visualize the Reflectivity Values in the Column**
+#
+# Let's visualize the reflectivity values in the column
+# above our point, which is stored in our new dataset
+ds.corrected_reflectivity_horizontal.plot(y='height');