LHCbMath: rewrite 2D/3D interpolation
- HistoInterpolation
- 2D & 3D case: implement all possible interpolation rules, up to biqubic and triqubic
- add possibility to interpolate for "density" ( "bin-content"/"bin-volume")
- add possibility for extrapolation
- better/uniform treatment of edge case
- fix some typos in doxygen