Refactoring using `get!`

#  get!(f::Function, collection, key)
#
#  Return the value stored for the given key, or if no mapping for the key is present, store key =>
#  f(), and return f().
#
#  This is intended to be called using do block syntax:
#
#  get!(dict, key) do
#      # default value calculated here
#      time()
#  end

# code from https://stackoverflow.com/q/54244661/1346276

function V2(t, K)
    if t >= T
        return 0.0
    else
        get!(V2dict, (t, K)) do
            opt = optimize(K′ -> -(log(K - K′) + β * V2(t+1, K′)), eps(), K, iterations = 100_000)
            Optim.minimum(opt)
        end
    end
end