Kernel rescaling

FMM3D v2.1.0

Changes from v2.0.0

• The kernels have been rescaled to include the factor of 4pi, e.g. the Laplace FMM now computes the sums \sum_{j}\frac{\sigma_{j}}{4\pi|x_{i} - y_{j}|} instead of \sum_{j}\frac{\sigma_{j}}{|x_{i} - y_{j}|}