Revise suspension model

Re-implement corrections to the model from !61 (closed).

• Upper joint temperature comes from the stage above
• Drop support for BQuad suspension type
• Use the fiber end cross-section consistently in wireTELoss()
• Remove the "questionable approximation" in continuumWireKh
• Correct the spring constant calculation in continuumWireKv

gwinc/test/ref_hash

-9ff4ba5463895698f48c243e329c4138ab163c1f
+06ec15931ae295717ac564ca892f2f92d33430c7