8.6.4 get_subscan_index(xsubscan_index,ii)

get_subscan_index $<$ f1 $<$ BoaF1.f90

! NAM: get_subscan_index (subroutine)
! DES: get the beginning index of each subscan data in phase array
!      by comparing the subscan begin time with lst_p.
!      Result must be fetched from f90.data module.
!      Might be good to specify the ending index as well if we sample 
!      data in between subscans.
! INP: data.subscan_time   f
!      data.lst_p          f
! OUT: data.subscan_index  i
! HIS: FB040429 created
!      FB040610 add second index to xsubscan_index