Add more interfaces so restrictions are more clear -> more typesafe

[Joelius300]

The idea is to make everything a bit more typesafe. Currently you can plug in any Type for TData in ChartConfigBase which allows basically anything to be the Data which will be serialized. This isn't a big problem because it's not exposed to the user of the library (unless they implement a new chart).
However currently the Data for bar- and linecharts use the same interface for their Datasets. This means that you could use a dataset for a barchart in a linechart. This should only allowed in a mixed chart. For this reason it would be good to have more abstractions. Think something like this:

• IDataset <- covarient, defines the IReadOnlyCollection and the ChartType
• IMixableDataset <- implements IDataset
• ILineDataset <- implements IMixableDataset
• IBarDataset <- implements IMixableDataset
• IBubbleDataset <- implements IDataset<BubblePoint>
• DoughnutDataset <- implements IDataset

For this solution it might also be a good idea to have something like INumber which is implemented by all number wrappers. You could also just make all the numbers to floats because you can still store a 2 in a float.

There might be a better solution for this but this is one of the floating ideas I had about it.

[Joelius300]

Another possibility would be to have a base-interface for every type say ILine and IBubble etc (which implement IChart). Then you would have

class BaseChart<TType, TOptions, TData> 
where TType : IChart 
where TOptions : BaseOptions, TType 
where TData : BaseData, TType

You could then create your chart like class LineChart<ILine, LineOptions, LineData>
and most restrictions would already be done. This would also make everything more typesafe but I don't know how good of a solution this would be.

[SeppPenner]

For me, the second solution seems to be good. It's a lot better than now.