fix: cell dectection for zero lattice length (#53)

* fix: non-pbc handle

* fix: zero cell detect

* fix bugs

* fix: cell detect error when cell is negative

* fix bugs

---------

Co-authored-by: Xixian <v-xixianliu@microsoft.com>

src/mattersim/datasets/utils/convertor.py

@@ -206,7 +206,7 @@ def convert(
                 atoms.set_cell(lattice_matrix)
                 atoms.set_pbc(pbc_)
             else:
-                if np.all(atoms.cell < 1e-5):
+                if np.all(abs(atoms.cell) < 1e-5):
                     raise ValueError("Cell vectors are too small")
         else:
             raise ValueError("structure type not supported")