Fritzing uses more than 2 digits for tool in their Excellons. To comply
with that, I check specifically for 3 or less digits and use as tool
number, more than that we treat as the standard (2 for tool and 2 for
compensation index)
CAUTION: the original code used zero_suppression flags
in the opposite sense as Gerber functions. This
patch changes it to behave just like Gerber code.
* Add metric/inch conversion support
* Add settings context variable to to_gerber just like Gerber code.
* Add some missing Excellon values.
Tests are not entirely updated.
