Fixed the handling of type-wise cutoff factors
This fixes the placeholder *_typewise_cutoff_factor fields on nep.model.Model
flagged in issue #XX. radial_typewise_cutoff_factor and
angular_typewise_cutoff_factor never corresponded to a real field in GPUMD's
nep.txt for NEP potentials (they were orphaned by an earlier refactor that
repurposed those cutoff-line slots for per-species typewise cutoffs) and have
been removed. zbl_typewise_cutoff_factor does correspond to a real, optional
fourth value on the zbl line (zbl rc_inner rc_outer [zbl_factor], used when
use_typewise_cutoff_zbl is enabled during training) and is now properly parsed
by read_model and round-tripped by Model.write.
Closes #138 (closed)