ZIP - now handles tar archives as well as compression/decompression utilities

I've added tar_append and tar_extract operations to the ZIP XOP to handle tar archives in IGOR. These features are only present in the OS X version. All other functionality is still in the Windows version namely:

ZIPdecode - compress a string
ZIPencode - decompress a string (can be used to compress images/waves)
ZIPfile - decompress a .zip or .gz or .z file
ZIPzipfiles - compress files into .zip or .gz or .z file.