generate compressed .mag files itself, it is often very useful to
compress a massive layout file to save disk space or to hit a github
size target. The code change looks for files with either ".mag" (default)
or ".mag.gz" extensions. No check is made to see if both files might
exist at the same time and have incompatible content; it is up to the
end user to manage the file compression.
R. Timothy Edwards
00c692b140