Function Append a string to the named file. The file is created if it does not already exist.
Function returns True upon success and False upon failure.
Checksum is returned as a one-character string. Uses modulo addition so that the function will return the 8 least significant bits of the sum. Uses modulo addition so that the function will return the 16 least significant bits of the sum. Uses modulo addition so that the function will return the 32 least significant bits of the sum. Checksum is returned as a one-character string.