Correct usage of unknown values in dREL code

Author: vaitkus

cif_pow.dic

@@ -14,7 +14,7 @@ data_CIF_POW
     _dictionary.title             CIF_POW
     _dictionary.class             Instance
     _dictionary.version           2.5.0
-    _dictionary.date              2025-07-15
+    _dictionary.date              2025-07-30
     _dictionary.uri
 https://raw.githubusercontent.com/COMCIFS/Powder_Dictionary/master/cif_pow.dic
     _dictionary.ddl_conformance   4.2.0
@@ -2488,7 +2488,7 @@ save_
 save_pd_calib_d_to_tof.coeff
 
     _definition.id                '_pd_calib_d_to_tof.coeff'
-    _definition.update            2022-09-30
+    _definition.update            2025-07-30
     _description.text
 ;
     The value of the coefficient used in the equation to convert
@@ -2519,15 +2519,15 @@ save_pd_calib_d_to_tof.coeff
     else if (pc.power == -1) {
         _units.code = "angstroms_per_microsecond"
     }
-    else _units.code = UNKNOWN
+    else _units.code = ?
 ;
 
 save_
 
 save_pd_calib_d_to_tof.coeff_su
 
     _definition.id                '_pd_calib_d_to_tof.coeff_su'
-    _definition.update            2023-07-11
+    _definition.update            2025-07-30
     _description.text
 ;
     Standard uncertainty of _pd_calib_d_to_tof.coeff.
@@ -2558,7 +2558,7 @@ save_pd_calib_d_to_tof.coeff_su
     else if (pc.power == -1) {
         _units.code = "angstroms_per_microsecond"
     }
-    else _units.code = UNKNOWN
+    else _units.code = ?
 ;
 
 save_
@@ -14423,7 +14423,7 @@ save_
 
        Deprecated _pd_refln.wavelength_id after consultation with PDDMG.
 ;
-         2.5.0                    2025-07-15
+         2.5.0                    2025-07-30
 ;
        ## Retain above version number and increment date until final
        ## release