Add multiple outputs for the evaluation of Sobol indices

[andreaperin]

- the 'output' argument in the 'sobolindices' funcition have to be a vector

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.16 :tada:

Comparison is base (97075ba) 98.92% compared to head (5d0393a) 99.09%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #98 +/- ## ========================================== + Coverage 98.92% 99.09% +0.16% ========================================== Files 21 21 Lines 652 663 +11 ========================================== + Hits 645 657 +12 + Misses 7 6 -1 ``` | [Impacted Files](https://codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf) | Coverage Δ | | |---|---|---| | [src/sensitivity/sobolindices.jl](https://codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/98?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf#diff-c3JjL3NlbnNpdGl2aXR5L3NvYm9saW5kaWNlcy5qbA==) | `100.00% <100.00%> (ø)` | | ... and [2 files with indirect coverage changes](https://codecov.io/gh/FriesischScott/UncertaintyQuantification.jl/pull/98/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jasper+Behrensdorf)

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[FriesischScott]

It's probably a good idea to save the output values in a Matrix and then do the computations for all from there.