Minor Fixes - New Dynamic Parts

Changes

Small changes that should have only a minor impact on existing signal declarations

Signal

https://github.com/HiceS/mirabuf/blob/7d0b2571a44ead67a8eba61c1697532063861b76/signal.proto#L43-L49

As discussed with @KyroVibe and @HiceS we decided that Signal message didn't make much sense. We have tried to keep it as faithful to the original as possible however we are changing the device_type to only be an enum now. However the old device type will now by used as a custom type field to ensure the files can still be loaded. However they might not function identically.

Motor

Changed units on DCMotor::stall_torque from in-oz to Nm due to popular demand and common sense

PartDefinition

After looking through the old ability to export a field we wanted an easy way to nest simple dynamic rigidbodies inside of a static assembly without entirely isolating them.

The easiest way is to add 3 new fields to the PartDefinition that can propagate out to the instances :

bool dynamic = 5;
float friction_override = 6;
float mass_override = 7;

Now we can nest balls and other simple objects within a single static assembly.