GSoC 5th week review

In this week, I continued the previous work on defect control adaptivity and added all of the BVP test problems and the others to the DiffEqProblemLibrary.jl, and closed the relating issue. All of the work in this week can be summarized as follows:

1. Fixed the indexing on discrete stages k_discrete and interpolation coefficients k_interp. The original indexing method for k_discrete and k_interp is awkward, by using a new matrix structure, we can directly use k_discrete[ith_subinterval, ith_stage] and k_interp[ith_subinterval, ith_stage] to addressing discrete stages and interpolantion coefficients.
2. Fixed the new stages and solution addition in the routine sum_stages. After we scaled the new stages using dt, we need to add the new stages to the solution in the specific interval.
3. Add the nonlinear BVP test problems to DiffEqProblemLibrary.jl
4. Add the regular nonlinear BVP test problems to DiffEqProblemLibrary.jl

Next week, I will continue to work on the problem that defect_norm is always bigger than the threshold, by comparing with the MIRKDC routine and monitoring the computing flow, while the other routines are the same, I think the problem lies in the collocation part.