refactor: rename OPENMX2DeePTB to OPENMX2DFTIO for consistency

Updated variable name in constants.py to align with the project's naming
convention and reflect the transition from DeePTB to DFTIO. This change
improves code clarity and maintains consistency across the codebase.

Author: QG-phy

dftio/constants.py

@@ -84,7 +84,7 @@
     "f": [0, 1, -1, 2, -2, 3, -3]
 }
 
-OPENMX2DeePTB = {
+OPENMX2DFTIO = {
             0: np.eye(1, dtype=np.float32),
             1: np.eye(3, dtype=np.float32)[[1, 2, 0]],
             2: np.eye(5, dtype=np.float32)[[2, 4, 0, 3, 1]],