ENH: Add command line parameter to average overlapping pixels.

This can be used to compare the results of averaging to the
blending algorithm implemented in MergeAdjacentImages.

Change-Id: Ia0567d4a687aa788c3921064101ef16fbf63d92c
Merge pull request #161 from chrismullins/average-overlapping-pixels-option

ENH: Add command line parameter to average overlapping pixels.