Add a bunch more docstrings to problems and solutions

[ChrisRackauckas]

QuadratureProblem and OptimizationProblem will need more work, but we can keep that as an issue for the future.

[codecov]

Codecov Report

Merging #157 (ed01267) into master (a3ea3c5) will increase coverage by 4.21%.
The diff coverage is 30.92%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   12.76%   16.97%   +4.21%     
==========================================
  Files          40       40              
  Lines        3009     3016       +7     
==========================================
+ Hits          384      512     +128     
+ Misses       2625     2504     -121     

Impacted Files	Coverage Δ
src/solutions/dae_solutions.jl	0.00% <ø> (ø)
src/solutions/nonlinear_solutions.jl	0.00% <0.00%> (ø)
src/solutions/optimization_solutions.jl	0.00% <0.00%> (ø)
src/solutions/rode_solutions.jl	0.00% <0.00%> (ø)
src/problems/basic_problems.jl	15.62% <13.63%> (ø)
src/solutions/ode_solutions.jl	56.52% <54.34%> (ø)
src/solutions/basic_solutions.jl	55.55% <100.00%> (ø)
src/utils.jl	58.13% <0.00%> (-0.90%)	⬇️
src/ensemble/basic_ensemble_solve.jl	40.20% <0.00%> (-0.42%)	⬇️
src/scimlfunctions.jl	6.95% <0.00%> (+0.23%)	⬆️
... and 5 more

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