Make coefficient recurrence precomputation / derivative taking smarter

for Taylor expansions.

* Introduces a DerivativeWrangler class that manages derivatives for
  Taylor expansions

* Closes #4 on gitlab: Precomputing the recurrences for Taylor
  coefficients is faster

* 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)

* Closes #6 on gitlab: This teaches the M2L step about recurrences.