Fix test spec for charts when running on Firefox

Firefox throws an error in tests that call `window.getComputedStyle'.  As these tests
are not intended to check the call to `getComputedStyle`, a spy has been introduced.
Improve handing of dirty data and error logging for correlation
Add create property accessor function under a new helper namespace
Add error container helper class
Improves ErrorContainer documentation and updated correlation tests
Improve stability of average calculations in grouping
Create error message constants
Merge pull request #120 from matthewdunsdon/ImproveRobustnessOfCorrelationCalculations

Improve robustness of correlation calculations
Merge branch 'upstream' into develop