Fix Gcode viewer and estimates when used with Relative extrusion
Change Z_offset centering to use the object center
Add missing exception on error messages.
New math to calculate the optimal extrusion width based on nozzle diameter, die swell, layer height and desired shell thickness.

Uses the nozzle diameter + die swell to determine maximum extrusion width.
Uses the maximum width from above to calculate the maximum layer height.
Uses the desried shell thickness / layer size to calculate the required solid layer count.
Uses the desired shell thickness / calculated extrusoin width to calculate the perimeter count.
Revert - "Change Z_offset centering to use the object center" to use machine_center or half of machine width.