Nllgrid

- `NLLGrid.get_value()`: support for 2D grids
- `NLLGrid.plot()`: support for 2D grids
- Fixed: `NLLGrid.get_value()` now correctly returns angles,
instead of `None`, for `ANGLE` and `ANGLE2D` grids

- `NLLGrid.iproject()` method for inverse projection

- Move to `python-versioneer`

- Fixes for PyPI packaging

- Installation instructions via pip

- Bug fixes