Refactor of pi_3_model_b_holes

[thecarp]

In order to support the creation of board mounts, these changes refactor the raspberry pi hole mount functions to allow them to array child blocks to each of the mount hole positions or fall back to the original behavior of producing M2.5 standard fit holes.

example to put a transparent bolt in each hole: board_raspberrypi_3_model_b(); raspberrypi_3_model_b_holes() %cylinder(h=10, center=true, r=1.25);