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