p3dfft 2.7.5

This release combines the two versions into one package: use the --enable-openmp flag to the configure script to switch between regular P3DFFT and Hybrid MPI-OpenMP (multithreaded) P3DFFT.

New in 2.7.5:

- Multithreaded support (MPI/OpenMP) - OpenMPI support (as option for MPI layer in configure script) - C++ compatibility - pruned transform test - bug fixes