Keyword symbol, one of :inches, :millimeters, :feet, :miles, :meters, :kilometers,
Mils:
:microns, :centimeters, :microinches, or :no-units-specified. This determines the units when outputting geometry.
This value can be overridden with the :units keyword to the with-writer (for IGES, and soon STEP and other formats). Default
value is :inches.