Moved coordinate conversion method from a category

Although I like categories a lot, it seems that they're a bit difficult
for MonoTouch people to copy. If we move this method out of a category
on SChartAxis then it'll be a lot easier to translate.
Updated the momentum animation so that it works with Charts 2.2.0

The way animations are done has changed between 2.1.0 and 2.2.0. Rather
than having a single class to evaluate animation curves based on an
enum, we now have an SChartAnimationCurve protocol, and some concrete
implementations of different animation curve types. This commit brings
the code in line with 2.2.0.