sippi_forward_gpr_fd: full waveform gpr forward
 
  Call :
   [d,forward,prior,data]=sippi_forward_gpr_fd(m,forward,prior,data,id,im)
 
   the prior must be such that m{1} relfect the eps field, and (optionally)
   m{2} reflect the sig field (if not set it is trated as constant).
  
  % Mandatory 
  forward.sources and forward.receivers contains the position of sources
     and receives ising the SAME coordinate systrem as prior{im}
 
  forward.sources : [Sx1 Sy1
                     Sx2 Sy2
                     ..]
  forward.receivers: [Rx1 Ry1
                     Rx2 Ry2
                     ..]
 
 
  forward.ant_pos contains the positions in a grid with origin [0,0]
                This is computed from forward.sources and forward.receivers
                if not set.
  forward.ant_pos: antenna postions for receivers and recorders
                   [Sx1, Sy1, Rx1, Ry1    
                    Sx2, Sy2, Rx2, Ry2
                    ..];
 
  ! Note that idential source locations muyst be located in neighboring row
  in forward.ant_pos
 
 
  Optional settings
  forward.T=100*10^-9; % Frequecy of source wavelet
  forward.sig=3; % If not set it is set as a constant field
  forward.output_type='shot'; % each shot gather is output as individual data structures
  forward.output_type='trace'; % each trace is output as individual data structures
  forward.output_it=10; % only output every 'output_it' time sample
 
 
  forward.dx_forward % the spatial sampling used for FDTD modeling
                       If this is different than the spatial sampling
                       density for the prior, then prior realizations are
                       rescaled before forward modeling
 
 
  % Settings for forward modeler
  All seetings for the forward modelling code can be set in the 
  forward.addpar structure. E.g to set the number of threads used:
  forward.addpar.cores=4;%
 
 
  See also FDTD_fwi