combine suspQuad and suspBQuad functions
These two functions were basically identical except for the material used for the test mass stage, and a hacky reduction of the spring constant for silicon. suspQuad is modified to support different materials for the test mass stage, and a hacky switch is added to adjust the spring constant. suspBQuad is now just a wrapper around suspQuad specifying that the test mass material is silicon instead of silica (there is definitely a better way to do this, by e.g. specifying the fiber material in the Stages struct list).