Write a backmapping trajector where anisotropic sites are converted into multi-site models.
Arguments
sites: comma-separated list of names of anisotropic sites to backmap.
fstprts: comma-separated list of particle files to backmap anisotropic sites into, with same length as the sites argument.
FileVMD, FileXYZ and Stepper arguments.
append is always set to true via Stepper:set_append().