Returns the floating point number represented by the eight bytes at
byteOffset in this object, in IEEE 754
double-precision binary floating-point format (binary64).
Throws RangeError if
byteOffset is negative, or
byteOffset + 8 is greater than the length of this object.
double getFloat64(int byteOffset, [Endianness endian = Endianness.BIG_ENDIAN]);