Feature plot notebook

**Summary:**  This pull request
* Adds the `SuperDARN Data Plotting.ipynb` notebook, which demonstrates RTI, Fan, and Convection Plotting.
* Addresses the default channel bug by setting `channel=None` in all calls.  This bugfix is needed to make this new notebook work properly.
* Somewhat addresses a bug which arises when the user does not have access to the remote MongoDB radarInfo database and does not have the SD_RADAR and SD_HDWPATH enviroment variables set and pointing to the radar.dat and hdw.dat locations.

**Testing:** Please run the `SuperDARN Data Plotting.ipynb` notebook.