minor bugfix for GS model FOV overlays

**Summary:** The ground scatter (GS) model is not defined for close-in range gates.  Therefore, FOV objects return np.nan for lat/lons of undefined rangegates.  This case was not handled properly in pydarn.plotting.overlayFov().  This pull request fixes that issue by setting the sGate to greater than the user specified value when the user-specified value would otherwise be invalid.  This pull request only affects pydarn.plotting.overlayFov() and should not interact with other parts of davitpy.

**Example:**

```python
from matplotlib import pyplot as plt
import datetime
import pydarn
import utils

time   = datetime.datetime(2012,11,15)
lat_0  =  90.0 
lon_0  = -85.0
width  = 45000000
height = 45000000

fig = plt.figure(figsize=(10,8))
ax = fig.add_subplot(111)
m = utils.plotUtils.mapObj(projection='stere',datetime=time,
                           width=width,height=height,
                           lon_0=lon_0,lat_0=lat_0,ax=ax)
pydarn.plotting.overlayFov(m,codes='fhe',dateTime=time,model='GS')

plt.show()
```

Currently on `develop`:
![image](https://cloud.githubusercontent.com/assets/1831182/4682800/ecabcef0-5620-11e4-9e9c-9faa27833456.png)

After this pull request:
![image](https://cloud.githubusercontent.com/assets/1831182/4682815/08a7449a-5621-11e4-96bc-0b7ddf541e8b.png)