Gravity std::pow() update

[ryanstoner1]

Resolves #4485.

Replaces pow(x,2) and pow(x,3) with multiplication for speedup. Tests from #4687 should apply.

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[bangerth]

Thanks!

For reference, when you write patches, you don't need to merge the main branch into your development branch. github will automatically make sure nothing gets lost.

[bangerth]

/rebuild

[ryanstoner1]

Thanks!

For reference, when you write patches, you don't need to merge the main branch into your development branch. github will automatically make sure nothing gets lost.

@bangerth Great, thank you for the pointer!