Add boundary optimized operators of Mattsson, Almquist, and van der Weide

[ranocha]

See https://doi.org/10.1016/j.jcp.2018.06.010 and https://bitbucket.org/martinalmquist/optimized_sbp_operators/src/master

• MattssonAlmquistVanDerWeide2018Minimal
  • Interior order 4
  • Interior order 6
  • Interior order 8
  • Interior order 10
  • Interior order 12
• MattssonAlmquistVanDerWeide2018Accurate
  • Interior order 4
  • Interior order 6
  • Interior order 8
  • Interior order 10
  • Interior order 12

[xlxs4]

I can give this a shot if it's still relevant

[ranocha]

Sure, would be great 👍