Bypass low-sensitivity IFOs in postcoh trigger formation;
 the bypassed IFOs will be used to find corresponding SNRs for postcoh triggers.

Code changes:
Online pipeline interface change (gstlal_inspiral_postcohspiir_online):
add one option --cuda-postcoh-parti-ifos to indicate participating IFOs in postcoh, e.g. H1L1
(default). The left-out IFOs from the option --iir-bank setting will be bypassed and
only used for SNRs. E.g. --iir-bank setting: --iir-bank H1:H1bank,L1:L1bank,V1:L1bank,K1:K1bank.

One new plugin to attach SNRs from less sensitive IFOs to the postcoh trigger: trigger_jointer.

Pipeline assembly code changed to put this new plugin in the right place.