Refactoring and New

1. Refactored some components to better align with latest Swift language trend by jjatie.
2. Add a new DataApproximator+N #2848 by 666tos.
3. New unit tests for CombinedChartView and demo builds checker.

We are making Swift refactoring recently and target for 4.0 release along with some other features, thanks everyone for improving this project!

Bug fixes

to name a few that may impact users, others please refer change log:

1. #2257 HorizontalBarChart does not update labels correctly
2. #3132 Fix axis label disappear when zooming in deep enough
3. #3149 Subclassing of LegendRenderer didn't take any effect
4. #2049 fix the issue with axis dependencies in CombinedChartView
5. Fix some Swift 4.0.3 errors to support Xcode 9.2