void ToHexString(std::string &str) const
The ToHexString method generates a hexadecimal string representation
of an OEBitVector. Each `nibble' of four bits is converted into an
uppercase hexadecimal character. Hexadecimal strings can only represent
bitmaps that have a multiple of four bits. For OEBitVectors that are
not a multiple of four, the remaining bits of the most significant nibble,
i.e. the first character, are assumed to be zero.