Add barnes hut algorithm, benchmark framework and minor API changes

[adityapb]

ToDo before merge

• Update docs for the changes in API
• Add docs for barnes hut algorithm
• Add gravity softening to barnes update
• Update docs for gravity softening

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Repository health decreased by 6% when pulling d76befa on barnes into b9e53bd on master.

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Repository health decreased by 6% when pulling be284b6 on barnes into b9e53bd on master.

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Repository health decreased by 6% when pulling 6a83dcb on barnes into b9e53bd on master.

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Repository health decreased by 6% when pulling e34391d on barnes into b9e53bd on master.

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Changes Unknown when pulling e34391d on barnes into * on master*.

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Repository health decreased by 6% when pulling add502f on barnes into b9e53bd on master.

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Repository health decreased by 6% when pulling ff5f198 on barnes into b9e53bd on master.

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[landscape-bot]

Repository health decreased by 6% when pulling ae5a814 on barnes into b9e53bd on master.

• 12 new problems were found (including 6 errors and 6 code smells).
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