Fixed upsie with bom generator, new spring generator

[maidenone]

• fixed upsie where BOM crashed when row was empty
• added spring generator
• fixed nut diam to correlate to max values within the standard
• removed nut_inner / nut outer dia as most nuts only have one dia
• fixed nut size bug where dia was set as radius
• added more faces to screw pins and nut holes

[etjones]

Cheers!