Fix save_idxs for multi-indices

[ChrisRackauckas]

Fixes #351

[codecov]

Codecov Report

Merging #352 (79f5c57) into master (5d8e840) will increase coverage by 0.06%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #352      +/-   ##
==========================================
+ Coverage   78.74%   78.81%   +0.06%     
==========================================
  Files          11       11              
  Lines        1449     1449              
==========================================
+ Hits         1141     1142       +1     
+ Misses        308      307       -1     

Impacted Files	Coverage Δ
src/common_interface/solve.jl	82.00% <77.77%> (ø)
src/common_interface/integrator_utils.jl	73.75% <100.00%> (+0.70%)	⬆️

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