Erddap-python

- pip package issues fix.

- Relocation of getDataFrame method, to base class erddap_dataset, this can be used by griddap_dataset also.

- Refactor of private methods
- Improved docstring using pdoc
- Published API reference in [https://hmedrano.github.io/erddap-python/](https://hmedrano.github.io/erddap-python/)

- Pytests
- First pip package