Ternary-diagram

Update docstring.

- Huge update.
- Resolved 5 .
- Defined default parameters as constants.
- Moved and defined some functions in utils.py.
- Added docstring.
- Changed to save the already plotted x and y as instance variables. (Added append_x_y method)
- Changed the class name to make it easier to understand.
- Changed _test.py in accordance with the above.

Closes 6
Closes 7

Update README.

- When a TernaryDiagram instance is created so that multiple ternary diagrams can be drawn, it can take matplotlib.pyplot.figure and matplotlib.pyplot.axis objects as arguments.
- Changed the axis specification to be more specific.
- The position of the axis labels has been fine-tuned.
- Updated the example.
- Added an example on how to draw multiple ternary diagrams.