Derivative wrangler
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Introduces a DerivativeWrangler class that manages derivatives for Taylor expansions
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Closes #4 (closed) on gitlab: Precomputing the recurrences for Taylor coefficients is faster
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In addition, caches the precomputed recurrence at the class level so we don't have to do it over for both multipole and local (this helps for 3D where the precomputation is still somewhat slow)
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Closes #6 (closed) on gitlab: This teaches the M2L step about recurrences.