Add packages for testing
Reference ExtCore
Remove estimations from aggregation

- Rather than complicating aggregation functions like sum or max with the trying to estimate approximate ranges of the possible results, just stick to saying it's unknown if there is any uncertainty.
Create ExList average function

- Create basic unit test suite - better than fsx file for refactoring, not got
  a test runner installed for any nice frameworks yet so just using VS default.
Remove unused packages

- I hope to switch to xunit once I've got ncrunch back on my laptop