ligolw_burca_tailor:  don't forget to check for vetoed coincs
ligolw_burca_tailor, stringutils:  use glue.offsetvector for offset vectors
snglcoinc:  move checks for None params out of CoincParamsDistributions

- this should increase the performance of this code even though the
  total number of checks is unchanged in code paths that require it, but
  certainly in code paths that don't require it