Pass on idxs keyword argument from ensemble plots

[ErikQQY]

While testing locally, setting idxs=3 or idxs=(3, 5) still display all of the components

[codecov[bot]]

Codecov Report

Merging #493 (bf3d605) into master (73eb334) will decrease coverage by 0.44%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #493      +/-   ##
==========================================
- Coverage   57.00%   56.56%   -0.44%     
==========================================
  Files          50       50              
  Lines        3703     3707       +4     
==========================================
- Hits         2111     2097      -14     
- Misses       1592     1610      +18     

Files Changed	Coverage Δ
src/ensemble/ensemble_solutions.jl	43.36% <0.00%> (-1.60%)	:arrow_down:

... and 5 files with indirect coverage changes

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