Absorbing barrier where the "bottom" is not necessarily the 0'th node.

[jlperla]

Not going to support reflecting or mixed off of the bottom.

• [x] Math with a homogenous absorbing barrier at a higher grid location. By default it is at the 0.
  • For example, say that it was at the 2nd node instead of the 0th node,
  • I think that we just need to show the additional equations in the gaussian elimination.
  • Would have a few columns of zeros, etc.
• [ ] Document a simple example for the homogenous case
• [x] Affine boundary condition math just to check
  • Only support a "constant" value for the non-homogeneous absorbing barrier.

[chiyahn]

Need to use affineboundary condition functions, math will be updated accordingly soon