Add homogenous absorbing barrier to the package

[jlperla]

• [x] Add in a new struct Absorbing <: HomogeneousBoundaryCondition (Resolved by adb4432)
• [x] Remove abstract type InhomogeneousBoundaryCondition <: BoundaryCondition end for now (Resolved by c45de5f)
• [x] Test for (Absorbing(), Reflecting()) (Resolved by e76d9b1)
• [x] Implement extrapolatetoboundary for (Absorbing(), Absorbing()) (Resolved by 529e3f5)

-- (added from https://github.com/QuantEcon/SimpleDifferentialOperators.jl/issues/148#issuecomment-490260155)

• [x] Extend the https://quantecon.github.io/SimpleDifferentialOperators.jl/stable/#Solving-HJBE-with-absorbing-barrier-conditions-1 to solve with the operator (Resolved by b84368a)
  • Key here is that f matrix would have a fudged first element. Give the formula
  • Put this in a notebook, with the full example for S != 0
• [x] Convert the current example to only be for S = 0 and use the operator with the boundary conditions to solve. (Resolved by 3e31f80)

[jlperla]

• [ ] Extend the https://quantecon.github.io/SimpleDifferentialOperators.jl/stable/#Solving-HJBE-with-absorbing-barrier-conditions-1 to solve with the operator
  • Key here is that f matrix would have a fudged first element. Give the formula
  • Put this in a notebook, with the full example for S != 0
• [ ] Convert the current example to only be for S = 0 and use the operator with the boundary conditions to solve.