make indexing EnsembleSolution work

[oscardssmith]

The last test is still failing, but this is almost good to go.

There are also a few typo fixes from #463 that would have been caught by tests but the tests didn't finish before merging.

[codecov]

Codecov Report

Merging #464 (0c3f019) into master (32cae24) will decrease coverage by 0.82%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #464      +/-   ##
==========================================
- Coverage   52.12%   51.30%   -0.82%     
==========================================
  Files          47       47              
  Lines        3555     3559       +4     
==========================================
- Hits         1853     1826      -27     
- Misses       1702     1733      +31     

Impacted Files	Coverage Δ
src/ensemble/ensemble_solutions.jl	32.38% <75.00%> (-12.52%)	⬇️
src/ensemble/basic_ensemble_solve.jl	74.52% <100.00%> (+6.60%)	⬆️
src/ensemble/ensemble_problems.jl	72.72% <100.00%> (+18.18%)	⬆️

... and 19 files with indirect coverage changes

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[oscardssmith]

I think this is now ready to merge.