GSoC 6th week review

This week, I complete the work on defect control adaptivity, the MIRK4 defect control workflow is working now. There are still some problems with MIRK6 method for now, but I think it will be figured out very soon.

Besides, I also did some work on improving the bvp-solving ecosystem on SciML:

1. Improve the initial guess of TwoPointBVProblem for BVP solving in SciMLBase.jl #462
2. Add the BVP test problems docstrings for DiffEqDocs.jl #677