#59 Teach gerbv to understand Altium's Excellon precision hint

[Mitch Davis]

Hi there.

Gerbv isn't able to auto-detect the right format for Excellon files generated by Altium, so the scale is often wrong. Inspecting the Excellon file shows a comment of the form ";FILE_FORMAT=2:3" (or similar). This patch teaches gerbv about the format hint.

The patch works for me, but maybe it needs more work on the way it interacts with the "Edit file format" dialog.

[Sergey Alyoshin]

From Mitch Davis:

There are still Altium files where the scale is not set
correctly. The problem is that INCH,TZ (etc) resets the number of
decimals back to 4. I haven't fixed it properly yet.