Amplpy

- Add Python 3.8 support.

- Add optional parameter index_names to DataFrame.fromPandas.
- Add DataFrame.fromDict to load data from dictionaries.
- Allow setting entity values directly from dictionaries.

- Update internal library.
- Add AMPL.exportModel.
- AMPL.exportData now supports indexed sets.

- Improve AMPL.exportGurobiModel and AMPL.importGurobiSolution.
- Add verbose option to AMPL.exportGurobiModel.
- Add register_magics to register `%%ampl` and `%%ampl_eval`.

- Fix dll loading issue with python versions that come with conda.

- Fix issue with indexed sets (by updating the internal library).