Release version 0.4.0

Release version 0.4.0

### Added
- Bidirectional T3 planner based on two T2 planners.
- RRT\*-Connect algorithm based on [Klemm2015] paper.
- Optimization procedure based on *Remove Redundant Points* from [Lan2015].
- Variable steer step size to `st3` procedure.

### Changed
- Make `sample` procedure part of RRTBase.
- Make `cost` function part of RRTBase.
- Make `nn` procedure part of RRTBase.
- Make `nv` procedure part of RRTBase.
- Make `steer` procedure part of RRTBase.
- Refactor `gplot` module.
- T2 planner to include variable steer step size.
- Set T3 planner as default.

### Removed
- RRTBase planner destructor. See `base/rrtbase.cc` for more info.