Pykonal

Basic location functionality is working

- Update version number

Major refactoring of the code. Not backwards compatible.

1. Replaced *EikonalSolver.vgrid* and *EikonalSolver.pgrid* attributes with *EikonalSolver.velocity* and *EikonalSolver.traveltime*, respectively, which use the ScalarField3D class.
2. Relabeled *EikonalSolver.is_far*, *EikonalSolver.is_alive*, and *EikonalSolver.close* to *EikonalSolver.unknown*, *EikonalSolver.known*, and *EikonalSolver.trial*, respectively, to be consistent with terminology in White et al. (2020).
3. Added new Field3D, ScalarField3D, and VectorField3D classes to facilitate storing and sampling 3D fields.
4. Added new PointSourceSolver class to facilitate the common problem of a point source with refined near-field grid.
5. Lots of documenting.

- Some refactoring of code
- Use -1 to represent NULL value instead of np.nan which was causing insidious errors
- Account for 2π periodicity when interpolating in spherical coordinates
- Set EikonalSolver.vvp = EikonalSolver.vv when EikonalSolver.vgrid == EikonalSolver.pgrid

- Debug EikonalSolver._get_gradient() which was returning None for Cartesian coordinates.
- Migrate demo notebook to current API

Improve stability by dropping imaginary part of solution to quadratic equation. This was previously done only when the discriminant was small negative, but now is done even when it is large negative.