8.6.3 compress(xdata,xflag,flag_value,ii)

compress $<$ f1 $<$ BoaF1.f90

! NAM: compress (subroutine)
! DES: compress array based on mask
! INP: data_input (f): 1-D array with data to be returned where flag = flag_value
!      flag       (i): 1-D array, must be same size as data_input
!      flag_value (i): select data where flag=flag_value
! OUT: tuple (x,ii): 1-D array of same size as data_input where first n elements are
!                   those of data_input where flag = flag_value  
! HIS: FB040430 created
! USE: 
!      Unfortunately it is not possible to pass from fortran to python an output
!      array the size of which is computed in fortran. Therefore we must truncate
!      the returned array at the size of the selected true mask elements.
!
!      Example:
!            input_array = array(range(5),'f')
!            flag_array  = array([0,1,0,1,0],'i')
!            compress_array,nmax = f90.f1.compress(input_array,flag_array,1)
!            compress_array = compress_array[0:nmax]
!            print "nmax compressed:",nmax,compress_array