Implementation of the absorbing (not at the boundary)

[jlperla]

After #190

Basically it is an absorbing where we pass in the node location of the boundary. Right now it is

struct Absorbing <: HomogeneousBoundaryCondition end

Perhaps better if it is

struct Absorbing <: HomogeneousBoundaryCondition
   loc::Int
end

with the default as loc = 0 in the constructor, which is our current code.

--

• [x] Implementation
• [x] Unit tests
• [x] Package jldocs
• [x] Example in main docs (Resolved in 66a3947)

[chiyahn]

Implementation complete in #198 (with affine boundary), will merge once a PR for #190 is merged (need some fix for math)