Fix a performance problem with adding RVs

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 84.81%. Comparing base (1bdbc78) to head (333ac69).

Files	Patch %	Lines
src/mcs/montecarlo.jl	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #986 +/- ## ========================================== + Coverage 84.77% 84.81% +0.03% ========================================== Files 40 40 Lines 4058 4062 +4 ========================================== + Hits 3440 3445 +5 + Misses 618 617 -1 ``` | [Flag](https://app.codecov.io/gh/mimiframework/Mimi.jl/pull/986/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mimiframework) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/mimiframework/Mimi.jl/pull/986/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mimiframework) | `84.81% <83.33%> (+0.03%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mimiframework#carryforward-flags-in-the-pull-request-comment) to find out more.

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