hides the parent traits of Chart, doc improvement

EncodableChart, WritableChart, DisplayableChart and StorableChart are
now package private.

- @usecase docs for most of the methods provided by these traits

- Chart itself now includes the class docs of DisplayableChart and
  StorableChart under separate headers

- Chart companion contains defaults like image resolution

- DisplayableChart.toComponent added in place of chart.toFrame -- a
  chart should be used as a Component in Swing, not as a container
PieChartLike now hidden from public API
getting to know API section in main package object