Base code for v2.2

Added support for MPI!
Additional testing and documentation required.
Code cleaning

Some int vs uint comparisons made explicit.
Minor bug fix
MPI: new barriers to reduce useless processing
v2.2 code ready

MPI support complete.
v2.2 docs

MPI documentation and minor changes in core docs.
v2.2 Mac binaries
v2.2 Linux binaries