Update ET to use ET-D noise curve
The ET interferometer object defaults to using the ET-B noise curve. However, the ET Collaboration requests the use of the ET-D noise curve for simulations, and ET-D has been the norm for a long time. The official reference for this noise curve is ET-0000A-18 in the ET document control system, but it is the same as the ET-D curve currently in the repository. This MR changes the default to use the ET-D noise curve.
Edited by John Douglas Veitch