API Changes requested

This covers a list of API changes suggested in the tablesaw repo. See original description below and original issue: https://github.com/jtablesaw/tablesaw/issues/716

I'm not sure what's left to do here that makes sense so a review is needed.

I would like to suggest some simple API changes to the jsplot-plotly classes which will slowly help in the direction proposed in issue https://github.com/jtablesaw/tablesaw/issues/693. If these suggestions sound acceptable, I can work on a PR.

Figure.java : Add methods .setTraces(Trace[]), .setLayout(Layout) & .setEventHandlers().

Figure.java : Change methods buildContext(), plotFunction(), eventHandlerFunction to "protected" instead of "private" so that they can be overridden.

For all Plot classes in tech.tablesaw.plotly.api.*, extend the classes to Figure.java.

Make the constructor of Heatmap (tech.tablesaw.plotly.api) public.

For all *Plot classes create constructors with parameters that match the static create() methods of each class, with an added Layout object. The constructors would do the same as the static methods except that it would setXXX() the layout, trace and eventhandlers instead of creating a new Figure.

If these changes are made, then the following will be true :

a) It will not break the current API.
b) It will allow users of Tablesaw to set a custom Layout object for each of the Plot classes.
c) It will allow users of Tablesaw to take advantage of inheritance and create classes which extend the Plot classes and override the underlying methods, so that they can make enhancements or changes to the current implementation.
d) It will allow users of Tablesaw to contribute their enhancements to the Tablesaw project, but still use their custom enhancements and the latest Tablesaw release while waiting for the acceptance of the enhancements and a new release with the changes.

Bắt đầu bằng cách xem xét Figure.java và các lớp Plot trong tech.tablesaw.plotly.api, sau đó đọc các issue gốc được liên kết 716 và 693 để xác định những thay đổi API nào vẫn còn phù hợp. Phần việc hoàn thành cần giữ nguyên API hiện tại đồng thời cho phép các đối tượng Layout tùy chỉnh và các lớp Plot có thể mở rộng, nhưng issue cần được xác định phạm vi và đưa ra các quyết định về thiết kế trước khi triển khai.