species/vectors: drop converter import, inline get_vector

The previous commit changed zmat.py to a module-style import of
vectors but the structural cycle (zmat -> vectors -> converter -> zmat)
remained, so CodeQL kept flagging both edges.

vectors.py only used converter in get_vector, where it called
xyz_to_x_y_z(xyz) (O(N) per-axis split + check_xyz_dict validation)
just to read two atoms' coordinates. Inline the two coord lookups
directly and drop the module-level converter import. The cycle is
now fully broken at the import-graph level.

Author: calvinp0

arc/species/vectors.py

@@ -9,7 +9,6 @@
 from arc.common import logger
 from arc.exceptions import VectorsError
 from arc.molecule.molecule import Molecule
-from arc.species import converter
 
 
 def get_normal(v1: List[float],
@@ -361,11 +360,9 @@ def get_vector(pivot: int,
     Returns: list
          A vector pointing from the pivotal atom towards the anchor atom.
     """
-    x, y, z = converter.xyz_to_x_y_z(xyz)
-    dx = x[anchor] - x[pivot]
-    dy = y[anchor] - y[pivot]
-    dz = z[anchor] - z[pivot]
-    return [dx, dy, dz]
+    px, py, pz = xyz['coords'][pivot]
+    ax, ay, az = xyz['coords'][anchor]
+    return [ax - px, ay - py, az - pz]
 
 
 def get_lp_vector(label: str,