make symbolic indexing return VectorOfArray

[oscardssmith]

This is a fairly minor change that makes things work more as expected.

[codecov[bot]]

Codecov Report

Merging #466 (7ff39ae) into master (f2eab6b) will increase coverage by 6.42%. The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
+ Coverage   43.47%   49.90%   +6.42%     
==========================================
  Files          47       47              
  Lines        3558     3561       +3     
==========================================
+ Hits         1547     1777     +230     
+ Misses       2011     1784     -227     

Impacted Files	Coverage Δ
src/ensemble/ensemble_solutions.jl	41.34% <0.00%> (-0.17%)	:arrow_down:
src/solutions/solution_interface.jl	46.93% <33.33%> (+17.85%)	:arrow_up:

... and 25 files with indirect coverage changes

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[ChrisRackauckas]

test?

[oscardssmith]

I think this is ready to go.

[ChrisRackauckas]

tests are failing.