Add the array overload to PROS 3

[Richard-Stump]

This is a feature we would like to implement in PROS4; however, MotorGroups have yet to be added to Pros 4, so they cannot be implemented to Pros 4. Since this feature is a small one that could be very useful for MotorGroups, it can be implemented into Pros 3 and easily ported into Pros 4. This would be fine to do since it is a small quality of life improvement, and we have #449 to push out when we get that fixed.

The gist of this feature is that if the user has a MotorGroup, they should be able to treat it like an array of motors:

pros::MotorGroup group({12, 13, -2, 3});

pros::Motor& motorA = group[3]; // motorA now references the last motor in the group.
pros::Motor& motorB = group[0]; // motorB mow references the first motor in the group.

The index passed should simply index into the list of motors that the motor groups.

Additionally, MotorGroups should have a method, size() which returns the number of motors in the group.

These features would allow for easy iteration over a motor group with a standard for loop, and without having to use many vectors for each of the properties needed.

for(int i = 0; i < group.size(); i++) {
      group[i].set_reversed(true);
      group[i].set_velocity(12);
}

This is redundant to be used like this, but this could maybe be used in more useful ways.

[noam987]

closed in #463