Updated microscope_stand for parametric wall thickness

This updated microscope_stand.scad includes new variables bottom_thickness and wall_thickness. Setting those values allows some adjustment for situations where printing using the default values is an issue. With ABS filament, users might find that wall_thickness=2.35 will print much more readily than the default of wall_thickness=1.5 (which seems to be working for PLA). Of course, users would need to modify the scad file appropriately, and then generate a printable format such as STL with the modifications.