#396 Dataset's ability to be cleared should be abstract over datasets

[Darker]

I noticed a strong inconsistency between Dataset's ability to clear all data. Some datasets define clear() method, time series defines removeAllSeries().

Since I never encountered a Dataset implementation that doesn't have some kind of clearing method, I propose that the Dataset's ability to be cleared is defined in the Dataset interface.

[David Gilbert]

The original thinking behind the Dataset interface was that it should define the minimal set of methods required to READ a set of data values. Modifying the data values was out of scope. I think it would be useful to have interfaces for writing dataset values, and that would help with consistency between dataset implementations...it is something to consider for JFreeChart 2.