clean up if-exists arg for write-tiff-file (now defaults to :error)

 * add remove-keyword-args and remove if-exists from args before calling write-tiff-stream
add &allow-other-keys to write-tiff-file
Deduplicate RGB compression code

In read-rgb-strip, merge codes dealing with different compressions
initial support for reading 16-bit compressed and uncompressed images

 * only works for big-endian images