new features:

spectral derivatives(curl)  for periodic greens function 
vorticity reprojection for periodic domains
freespace solution to poisson equation


git-svn-id: https://ppm.inf.ethz.ch/svn/ppmnumerics/branches/ngtopo/libppmnumerics@1025 7c7fe9aa-52eb-4d9e-b0a8-ba7d787348e9