Delta printer calibration test disc

To calibrate your delta style printer with Marlin, first determine the
```
DEFAULT_AXIS_STEPS_PER_UNIT
```
by printing an object of known height and measuring the result.

Afterwards, solve the formula
```
x = current_steps_per_unit * expected_height / measured_height
```
and x will be your new `DEFAULT_AXIS_STEPS_PER_UNIT`.

After fixing the Z axis dimensions, proceed to printing this thing.

Measure it across in the direction of your three towers to determine how much you have to trim each tower's diagonal rod length to get dimensionally correct prints.

If you find a measurement to be different from 100 mm, you may again use a formula to correct your printer's dimensions (where x is the number of the tower you measured against):

```
DELTA_DIAGONAL_ROD_TRIM_TOWER_x = DIAGONAL_ROD_LENGTH - (DIAGONAL_ROD_LENGTH * 100 / measured diameter)```
```

Example:

Your printer has a `DIAGONAL_ROD_LENGTH` of `255 mm` and you measured `102 mm` for tower number 1 ("X"). Th