Fan base for Parallella Cluster

The Parallella cluster stack packs 144 processors and 200 GFlops into a space of approximately 150x86x54mm. A veritable supercomputer on a desktop

While the Epiphany processors are very low power, the Zynq FPGAs hosting the ARM cores and glue logic get warm and need cooling.

This design provides a holder for two 120mm fans to force air across the FPGA heat sinks of the entire stack. The design is configurable (through `epiphany-paramaters.scad`) to allow for different size fans.

The ori