Add --average-min-count option to AverageAlgorithm

I hope you don't mind @awernle , but I went ahead and started this change we discussed because I need it in place to test some other ideas I have.

• 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.

requested review from @awernle

assigned to @erigler

approved this merge request

marked this merge request as ready

mentioned in commit ecff6d45

merged