There are three components to the radius error tolerance for center-format
arcs.

The "absolute error" is the difference between the radius at the starting
point and the radius at the ending point.  The "relative error" is the
absolute error divided by the maximum arc radius.

The absolute error threshold for imperial arcs is:

    2 * 0.001 inch * sqrt(2)

The absolute error threshold for metric arcs is:

    2 * 0.01 mm * sqrt(2)

The relative error threshold for all arcs is 0.001 (0.1%)

An arc fails the radius tolerance test if either of these conditions
is true:

    * The absolute error is greater than 100 * the absolute error
      threshold,

    OR

    * The absolute error is greater than the absolute error
      threshold AND the relative error is greater than the relative
      error threshold.


This yields three error regions:

    r_big < t_abs/t_rel (2.828 inch or 28.28 mm ): r_small > r_big - t_abs
    r_big < 100 t_abs/t_rel (282.8 inch or 2828 mm): r_small > (1-t_rel) r_big
    r_big > 100 t_abs/t_rel: r_small > r_big - 100 t_abs

