Added Greenbelt, MD to the list of cities.

This addresses Issue NCAR#5.  Greenbelt, MD is the home of NASA
Goddard Space Flight Center's Hydrological Sciences Laboratory.

Note that the mean, standard deviation, and number of day with
precip numbers are randomly modified from Philadelphia's, and
do not in any way represent actual values.

cities/greenbelt.json

@@ -0,0 +1,17 @@
+{
+    "name":     "Greenbelt",
+    "state":    "MD",
+    "high":     {
+        "avg":  19.1,
+        "std":  2.9
+    },
+    "low":      {
+        "avg":  5.3,
+        "std":  2.9
+    },
+    "precip":   {
+        "yamt": 1053.0,
+        "std":  25.0,
+        "days": 130.0
+    }
+}

wxgen.py

@@ -13,7 +13,7 @@
 from datetime import datetime
 
 # User settings
-city_list   = ["boulder","philadelphia"]
+city_list   = ["boulder","philadelphia","greenbelt"]
 fcst_path   = "forecasts/{}.txt"
 num_days    = 7