-------------------------------------------
variable: $dip_spacing -- dipole subsample spacing
-------------------------------------------
default: 1

  subsample_dist <$dip_spacing:(int)mm>
       or
  subsample_orient <$dip_spacing:1-adjdotprod>
  write_binary_decimation

If dip_spacing>0.99, dist subsample, else orient.
$dip_spacing updates decimation filename $dec.
