Add plotly pie charts

[guerler]

Add plotly pie charts to Galaxy, see: galaxyproject/galaxy-visualizations#125 for the visualization and galaxyproject/galaxy-visualizations#126 for the automated tests.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[bgruening]

Works like a charm! Will report back to the user ...