Add Method for Computing "Cross-overs" of Actuator Stages

(Apologies if this already exists; I think it did in pyDARM 1.0, but it may not have made it to a request for pyDARM 2.0.)

In order to characterize the design stability of the actuator stages' authority, a common tool is a "cross-over" measurement, where (with the whole DARM loop closed on all stages of the actuators) at a given stage's LOCK bank (stay the L2 stage), you drive an excitation in, and measure the response of that bank's IN1 / IN2. The math works out that this transfer function can be analyzed for stability like an open loop gain transfer function, but it's very much not a simple function of plants and distribution filters.

I think Evan did some work with Sheila when he was still at LHO to get pyDARM to make plots of these crossovers to compare against measurement, but I can't find any aLOGs or DCC entries about it. Looks like the code for the work lives in the pyDARM 1.0's svn folder, specifically

• Starting on Line 137 of
  trunk/Common/pyDARM/darm_loop_comparison.py
• Starting on Line 88 of trunk/Common/pyDARM/darm_loop_critique.py

But, I don't think it's made it into the "critique" method of plot.py, nor do I see it computed where I would expect it in actuation.py