BUGFIX: Correct VerifyChecksum method

Changed method to fuse the data with the corresponding
checksum at the end in a single raw bytes buffer. This
was done to satisfy crc64_omp usage requirements.
ENH: Shortcircuit if the data is NULL or nbytes==0
COMP: Add missing include for assert()
ENH: Use MPIUtilities printf functions
STYLE: remove commented out code