Rename 'inertia' to 'identity'

This was originally called 'I' and during the PEP8
conversion around the py2 to py3 time, they were 
all renamed inertia, even though some of them 
represent the identity matrix not the moments of inertia

Author: rwest

arkane/statmech.py

@@ -1078,13 +1078,10 @@ def project_rotors(conformer, hessian, rotors, linear, is_ts, get_projected_out_
             d[3 * i + 2, 5] = (p[i, 0] * inertia_xyz[2, 1] - p[i, 1] * inertia_xyz[2, 0]) * amass[i]
 
     # Make sure projection matrix is orthonormal
-
-    inertia = np.identity(n_atoms * 3, float)
-
+    identity = np.identity(n_atoms * 3, float)
     p = np.zeros((n_atoms * 3, 3 * n_atoms + external), float)
-
     p[:, 0:external] = d[:, 0:external]
-    p[:, external:external + 3 * n_atoms] = inertia[:, 0:3 * n_atoms]
+    p[:, external:external + 3 * n_atoms] = identity[:, 0:3 * n_atoms]
 
     for i in range(3 * n_atoms + external):
         norm = 0.0
@@ -1234,8 +1231,8 @@ def project_rotors(conformer, hessian, rotors, linear, is_ts, get_projected_out_
     # Do the projection
     d_int_proj = np.dot(vmw.T, d_int)
     proj = np.dot(d_int, d_int.T)
-    inertia = np.identity(n_atoms * 3, float)
-    proj = inertia - proj
+    identity = np.identity(n_atoms * 3, float)
+    proj = identity - proj
     fm = np.dot(proj, np.dot(fm, proj))
     # Get eigenvalues of mass-weighted force constant matrix
     eig, v = np.linalg.eigh(fm)