Evan, Stefan

We implemented the PR3 feed-back in ENGAGE_ASC in the ISC_LOCK guardian:
- We decided to leave the feed-back on PR2 during DRMI. This allows us to absorb the first correction to the initial alignment to PR2.
- We then switch the feed-back to PR3 in ENGAGE_ASC. This locks down PR3 during power-up - we confirmed that the REFL beam no longer moves in that transition.

- Details:
 - We modified the DRMI guardian ENGAGE_DRMI_ASC state to prepare both PR2 and PR3 for feed-back.
 - PR3 gais were set to roughly match PR2
 - The settings on PR3 are:
   -    ezca['SUS-PR3_M3_ISCINF_P_GAIN'] = 2
   -    ezca['SUS-PR3_M3_ISCINF_P_GAIN'] = 5
   -    SUS-PR3_M1_LOCK have an integrator, a -120dB and a gain of 1
 - Also, it now has the flag self.usePR3, which can select feed-back to PR2 (False) or PR3 (True). The rest of the ENGAGE_DRMI_ASC state uses this flag.
 - The default is self.usePR3=False, i.e. it does just the old PR2 engaging.

 - The PRC2 loop is always off during the CARM reduction sequence. It is re-engaged in ENGAGE_ASC with feed-back to PR3 with the following steps:
 - The output of SUS-PR2_M3_ISCINF_P and SUS-PR2_M3_ISCINF_Y is held
 - The ASC-PRC2_P and ASC-PRC2_Y filters are cleared.
 - The output matrix is updated:
   -    asc_outrix_pit['PR3', 'PRC2'] = 1
   -    asc_outrix_yaw['PR3', 'PRC2'] = -1 # required to keep the same sign as PR2
 - And the loops are turned on again.
 - The current loop gain is still low - the step response is on the order of 30sec.
 - ENGAGE_ASC also has the self.usePR3 flag (default is True), so it is still backwards compatible.

 - The whole sequence (engage DRMI on PR2, switch to PR3 in full lock) was tested successfully once before an earthquake hit.