COMP: Define __STDC_CONSTANT_MACROS for UINT64_C

CRC64 uses UINT64_C, which is a macro that appends the correct
suffix to a 64-bit unsigned integer literal. However, these
macros are not enabled in C++ by default. Therefore, C++ codes
need to specify the -D__STDC_CONSTANT_MACROS to enable those
macros.
ENH: Ensure data has checksums
ENH: Added CRC64CheckSum to GenericIOUtilities
ENH: Add CRC64 checksum for global header
ENH: Add CRC check for header
ENH: Rename method to VerifyChecksum
ENH: GIO handling of extra space for CRC

Don't clobber contents when GIO uses extra space in the
input/output buffers for CRC.
ENH: Add EntireHeader ivar to store the header

Added EntireHeader ivar to store the raw bytes of the
entire GenericIO header which includes the GlobalHeader,
VariableHeader(s) and RankHeader(s). This is primarily done
to facilitate checking of the checksum which is computed for
the entire header as opposed to individual pieces.
ENH: Move MPI communicator to GenericIOReader
ENH: Read entire header into bytes & distribute

Changed reader s.t. the entire header, including the
GlobalHeader, VariableHeader(s) and RankHeader(s),
are read in to a raw bytes buffer and distributed
in memory.
ENH: Move Ranks/NumRanks ivars to GenericIOReader
ENH: Update ReadVariableHeader to use EntireHeader

Changed ReadVariableHeader to extrac the desired VariableHeader
from the EntireHeader instead of doing a read in the file.
ENH: Update ReadBlockHeader to use EntireHeader

Changed ReadBlockHeader to extract the desired BlockHeader
from the EntireHeader instead of goind a read to a file.
ENH: Completed OpenAndReadHeader with checksums
ENH: Add checksum verification for each variable
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