I should add here that on #343, it would be worth putting in some tests that the magic numbers do what we expect.  e.g. draw something with the extreme-accuracy set, and then check that raising kvalue_accuracy leads to errors no larger than this on the drawK() image, and that raising xvalue_accuracy does the same for draw() (for an unconvolved object).  Similarly that maxK() and stepK() actually meet the definitions we set out for them.