Utility class that calculates a CRC64 checksum on a stream of bytes. Code was
based on some from BioPerl. Note that we use longs then cast them to avoid
the lack of an unsigned int in Java. Longs are 64-bit but we are only using
the bottom 32 bits. An int is 32-bit but encodes sign so we can get amusing
results if we don't allow for this.