Fix errors.

- In include/fmm_pts.txx, for particle FMM, set scaling factor to 1.0
  for non-scale-invariants kernels.

- In include/cheb_utils.txx, fix error in cheb_integ(). Output
  dimensions were transpose of the kernel dimension.

- In examples/src/fmm_cheb.cpp, for Biot-Savart example, fix sign of
  source density function.

- Minor changes and clean-up in include/kernel.txx