Bracket for MRServo model railroad switch machine

A convenient mounting bracket for [Iowa Scaled Engineering MRServo](http://www.iascaled.com/MRServo) servo-based model railroad switch machines (all variants after Dec 2017, and -2/-3 units before that). Holds both the control board and 9g servo in one convenient mount, with slotted screw holes for easy adjustment. Look at our other things for a MRServo-1 bracket for MRServo units before Dec 2017 and a "just a 9g servo" bracket.

The part was designed in Fusion 360, but I've included the original f3d as well as more open IGS and STP files.

I recommend a #8 1/2" lath screw for actually mounting the bracket to the layout benchwork. My preferred mounting method is to hold the assembled bracket up where it goes, mark the screw points, and then pre-drill them. I find it much easier to get the bracket screws accurately positioned this way.