- Update Stokes double-layer potential to symmetric part of Stokes-dipole.
    => To evaluate Stresslet, evaluate: Stokes-dipole + Laplace-gradient
- Update example1 for evaluating double-layer potential.
- Allow turning off symmetries with option -DPVFMM_NO_SYMMETRIES.
- Update configure script to detect quadruple precision.
- Option to turn profiling on/off.