Merge pull request #14 from kostrzewa/InterleavedNDTwistedClover_fatal_error

use fatal_error to cleanly finalize MPI in case of SPI errors
Merge pull request #13 from kostrzewa/MPI_thread_overlap

use better threaded comm/comp overlapping with MPI
Merge remote-tracking branch 'upstream/master' into InterleavedNDTwistedClover
make _THREAD_OVERLAP optional through the --enable-threadoverlap configure argument, default=no