Can this scratchVector be removed?

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In NumericalEngine.ts, this looks like an old code-review comment that was not addressed. According to git hiistory, it was added by @samreid for code review issue #88. I'm going to promote it to a TODO.

        // REVIEW: Can this scratchVector be removed?
        body1.forceProperty.value = body1.forceProperty.value.plus( scratchVector.set( force ) );
        body2.forceProperty.value = body2.forceProperty.value.minus( scratchVector.set( force ) );

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I'm guessing that the question is whether the use of scratchVector in these 2 lines can be removed. Not whether scratchVector can be deleted entirely. (@samreid if you recall, please clarify.)

@AgustinVallejo can't we just add and subtract force directly? Like this:

-        body1.forceProperty.value = body1.forceProperty.value.plus( scratchVector.set( force ) );
+        body1.forceProperty.value = body1.forceProperty.value.plus( force );
-        body2.forceProperty.value = body2.forceProperty.value.minus( scratchVector.set( force ) );
+        body2.forceProperty.value = body2.forceProperty.value.minus( force );

[AgustinVallejo]

I'm not entirely sure, since then I've forgotten the exact reason why we added scratchVector there. @jonathanolson can you chime in?

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Looking at the other uses of scratchVector, they look appropriate. scratchVector is used to avoid modifying the values of in the positions, velocities, and accelerations arrays.

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In the above commit, I removed the 2 uses of scratchVector, since I'm certain that they are unnecessary.

But to confirm that the result of the computations is the same, I fuzz tested with this bit of code (which was not commited):

        const saveForce = force.copy();

        // Verify that the computation for body1 is the the same with and without scratchVector.
        const body1Force1 = body1.forceProperty.value.plus( scratchVector.set( force ) );
        const body1Force2 = body1.forceProperty.value.plus( force );
        assert && assert( body1Force1.equals( body1Force2 ) );

        // Verify that the computation for body2 is the the same with and without scratchVector.
        const body2Force1 = body2.forceProperty.value.minus( scratchVector.set( force ) );
        const body2Force2 = body2.forceProperty.value.minus( force );
        assert && assert( body2Force1.equals( body2Force2 ) );

        // Verify that force was unchanged as the result of not using scratchVector above.
        assert && assert( saveForce.equals( force ) );

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@AgustinVallejo please review and close if OK.

[AgustinVallejo]

Looks reasonable, thanks!