Add --average-min-count option to AverageAlgorithm

- added a `min_count` keyword to the AvergeAlgorithm's __init__() method;
- added a --average-min-count option via the add_arguments() classmethod;
- set `self.min_count = arguments.average_min_count` in configure() method;
- refactored process() method to respect `min_count`, but now it defaults
  to a requirement that all inputs be valid (this modified a recent merge
  by @awernle that only required that any inputs be valid;
- modified AverageAlgorithm_test.py to properly assess things in light
  of the change to default behavior just mentioned.