Add subsolutions

[YingboMa]

Ref #901

[codecov]

Codecov Report

Merging #429 (7ba9fc8) into master (3583407) will increase coverage by 5.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #429      +/-   ##
==========================================
+ Coverage   52.97%   57.99%   +5.01%     
==========================================
  Files          46       46              
  Lines        3507     3523      +16     
==========================================
+ Hits         1858     2043     +185     
+ Misses       1649     1480     -169     

Impacted Files	Coverage Δ
src/ensemble/ensemble_solutions.jl	45.91% <0.00%> (-2.54%)	⬇️
src/problems/problem_utils.jl	23.59% <0.00%> (ø)
src/solutions/ode_solutions.jl	88.98% <ø> (+16.10%)	⬆️
src/solutions/pde_solutions.jl	0.00% <ø> (ø)

... and 25 files with indirect coverage changes

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