v0.4.0

Release version 0.4.0

### Added
- 3D grid extension.
- Uniform sampling. (Normal sampling is still default.)
- Compound extensions 18 - 33.
- Path optimization in backward direction.
- Goal found check for zone instead of Euclidean distance and max. angle
  extension.

### Changed
- Reformat lists in readme.
- Add `RRTExt3` to compound extensions 18 - 33.