Silicon Rod Ice Tray Holder/Stiffener

This helps stiffen up silicon rod ice trays. Its a customiser object, but stl is provided with the default values shown below:

```
/* Tray Holder Spec */
tray_holder_thickness = 1;// Thickness of this object
tray_holder_cell_count = 7; // Number of cells to hold up

/* Silicon Tray Spec */
gap_l = 10; // Gap Above Each Cell (mm)
gap_w = 7; // Gap Between Cells (mm)
ice_w = 17; // Ice Cell Width (mm)
ice_l = 75; // Ice Cell Length (mm)
ice_h = 20; // Ice Cell Height (mm)
silicon_th