Update SwiftChart with Animations

[norfred]

• Introducing animations both for Series and Areas, w/ related delegate's method
• Fixed a problem with identities signing (maybe it was just me?)
• Refreshed UI
• Better files organization

[gpbl]

@norfred thanks for the PR, and sorry for the late answer 😅

• I'm not sure about the animation regarding the negative areas, i believe that was the main reason I didn't implement it for the first release. The animation should consider a serie as an unique line, while now the line is split in different segments:

- I'd better animate the areas by sliding them from left to right instead of fading them in. This also may require to complicate the code a bit. - I'm not for adding new methods to the delegate: instead, i'd keep the setting for the animation duration in the `ChartSerie` class, as it happens for the appearance. I wonder though if it is really needed: isn't the `animationDuration` in the main class enough? I can't see a case where a dev would need different values for each serie.

[gpbl]

Oh i just realized in #3 i was opting for animating a mask over the chart. Maybe this could work better.