Imrove initial guess handling

[ErikQQY]

Fix #274

Now BVProblem/TwoPointBVProblem supports passing solution type as the initial guess.
Add continuation documentation to illustrate how to use the initial guess.

[github-actions]

Benchmark Results

	master	7689568...	master/7689568e92df71...
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK2()	6.59 ± 0.24 ms	6.77 ± 0.2 ms	0.973
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK3()	2.54 ± 0.15 ms	2.58 ± 0.13 ms	0.985
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK4()	0.894 ± 0.061 ms	0.916 ± 0.062 ms	0.977
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK5()	2.48 ± 0.39 ms	2.48 ± 0.42 ms	1
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK6()	1.07 ± 0.15 ms	1.08 ± 0.13 ms	0.988
Simple Pendulum/IIP/MultipleShooting(10, Tsit5; grid_coarsening = false)	1.96 ± 0.6 ms	2.01 ± 0.58 ms	0.976
Simple Pendulum/IIP/MultipleShooting(10, Tsit5; grid_coarsening = true)	3.57 ± 0.87 ms	3.7 ± 0.88 ms	0.966
Simple Pendulum/IIP/MultipleShooting(100, Tsit5; grid_coarsening = false)	0.0541 ± 0.0093 s	0.0569 ± 0.0084 s	0.952
Simple Pendulum/IIP/MultipleShooting(100, Tsit5; grid_coarsening = true)	0.0681 ± 0.011 s	0.0715 ± 0.012 s	0.952
Simple Pendulum/IIP/Shooting(Tsit5())	0.252 ± 0.081 ms	0.257 ± 0.082 ms	0.98
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK2()	0.0408 ± 0.0012 s	0.0393 ± 0.00079 s	1.04
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK3()	11.9 ± 0.28 ms	11.5 ± 0.59 ms	1.04
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK4()	3.46 ± 0.21 ms	3.39 ± 0.22 ms	1.02
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK5()	10.1 ± 0.89 ms	9.96 ± 0.85 ms	1.01
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK6()	3.66 ± 0.29 ms	3.65 ± 0.31 ms	1
Simple Pendulum/OOP/MultipleShooting(10, Tsit5; grid_coarsening = false)	3.51 ± 2.6 ms	3.61 ± 2.6 ms	0.972
Simple Pendulum/OOP/MultipleShooting(10, Tsit5; grid_coarsening = true)	6.22 ± 4.3 ms	6.43 ± 4.6 ms	0.968
Simple Pendulum/OOP/MultipleShooting(100, Tsit5; grid_coarsening = false)	0.0993 ± 0.0054 s	0.109 ± 0.0075 s	0.912
Simple Pendulum/OOP/MultipleShooting(100, Tsit5; grid_coarsening = true)	0.121 ± 0.014 s	0.13 ± 0.0066 s	0.924
Simple Pendulum/OOP/Shooting(Tsit5())	0.611 ± 0.042 ms	0.629 ± 0.061 ms	0.972
time_to_load	5.62 ± 0.13 s	5.92 ± 0.22 s	0.95

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).