Repeating the same idea as we did for frequency noise in aLog  31176
we measured the sensitivity of REFL_A_RF45_I, POP_A_RF9_I and POP_A_RF45_I to PRC length noise, and then used this transfer function to project the noise into DARM.
(REFL_A_RF9_I is used for the CARM loop -so there is no interesting signal there.)

Around 40Hz the three projections basically agree, and are a factor of 2 below the noise floor. Also, it seems that REFL_A_RF45_I would actually have a lower noise floor than POP_A_RF45_I. We should consider using it for PRCL.

FInally, we should implement a PRCL_feed-forward path.

The templates for the measurment are here:

/ligo/home/controls/sballmer/20161107/PRCLNoiseProjection.xml
/ligo/home/controls/sballmer/20161107/REFLPOPV4.xml