change method used to join layers together

[kaeverens]

right now, four bolts are used in the corners of the robot to join each layer together.

removing the bolts takes time, and once removed, all layers are separate.

we might sometimes want to take out just one layer and not have the others removed.

a better solution than bolts might be to add butterfly splines (a woodworking trick) to the sides of the layers, with a screw hole in the top end of each spline to hold it in place. the bottom end would be held in place naturally by the shape of the spline.

removing a layer then just involves removing the splines holding that layer in.

[kaeverens]

I'm using a simple inset with a bolt hole. I'll use a triangle shape on the bottom layer.