Finish DocBlock documentation

Some of the methods were missing, or had outdated, DocBlocks. Comments
now wrap at 80 characters, for improved readability.
Correct minor inconsistencies

- Some of the output functions did not return values (this depends on the
  implementation of the graphics library, so it is of questionable vaule
  anyway and may be removed in the future).

- The removeSlice() and outputPNG() methods are now defined by PieChart,
  there is no need to ovveride them.
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