Update Model to work with type-wise cutoffs

In the most recent version of GPUMD the handling of type-wise cutoffs for ZBL was improved. This led to a change in the format of the nep.txt file. As a result this test is currently broken for the ZBL case:

pytest -sv tests/test_nep_model.py::test_potential_read_and_write_case1

To fix the error it appears that the handling of type-dependent cutoffs in the Model class needs to be upgraded.

Related !214 (merged)