Added a usage example for the Average Algorithm

docs/algorithms/Average_usage.md

+Average Algorithm Usage
+========================
+
+The average algorithm takes data streams from multiple observatories
+over one channel and returns a single averaged data stream.
+
+
+### Example
+To compute the average of H over the observatories of HON and SJG
+
+```
+geomag.py \
+    --algorithm average \
+    --observatory HON SJG GUA \
+    --type variation \
+    --interval minute \
+    --inchannels H \
+    --outchannels H \
+    --starttime 2017-07-04T00:00:00Z \
+    --endtime 2017-07-04T00:10:00Z \
+    --input edge \
+    --output iaga2002 \
+    --output-stdout
+```
+
+### Output
+
+```
+ Format                 IAGA-2002                                    |
+ Source of Data         United States Geological Survey (USGS)       |
+ Station Name           USGS                                         |
+ IAGA CODE              USGS                                         |
+ Geodetic Latitude      40.137                                       |
+ Geodetic Longitude     254.764                                      |
+ Elevation              1682                                         |
+ Reported               HNULNULNUL                                   |
+ Sensor Orientation     HDZF                                         |
+ Digital Sampling       100.0 second                                 |
+ Data Interval Type     1-minute calculated                          |
+ Data Type              variation                                    |
+ # DECBAS               0       (Baseline declination value in       |
+ # tenths of minutes East (0-216,000)).                              |
+ # CONDITIONS OF USE: The Conditions of Use for data provided        |
+ # through INTERMAGNET and acknowledgement templates can be found at |
+ # www.intermagnet.org                                               |
+DATE       TIME         DOY     USGSH     USGSNUL   USGSNUL   USGSNUL|
+2017-07-04 00:00:00.000 185     29854.29  99999.00  99999.00  99999.00
+2017-07-04 00:01:00.000 185     29854.31  99999.00  99999.00  99999.00
+2017-07-04 00:02:00.000 185     29854.28  99999.00  99999.00  99999.00
+2017-07-04 00:03:00.000 185     29854.18  99999.00  99999.00  99999.00
+2017-07-04 00:04:00.000 185     29854.15  99999.00  99999.00  99999.00
+2017-07-04 00:05:00.000 185     29854.20  99999.00  99999.00  99999.00
+2017-07-04 00:06:00.000 185     29854.22  99999.00  99999.00  99999.00
+2017-07-04 00:07:00.000 185     29854.35  99999.00  99999.00  99999.00
+2017-07-04 00:08:00.000 185     29854.52  99999.00  99999.00  99999.00
+2017-07-04 00:09:00.000 185     29854.69  99999.00  99999.00  99999.00
+2017-07-04 00:10:00.000 185     29854.76  99999.00  99999.00  99999.00
+```