[core][mback_norm] fix configuration read from mback_norm

xas/core/process/larch/mback_norm.py

@@ -63,8 +63,8 @@ def process_spectr_mback_norm(spectrum, configuration, overwrite=True,
         return None, None
 
     _conf = configuration
-    if 'mback' in _conf:
-        _conf = _conf['mback']
+    if 'mback_norm' in _conf:
+        _conf = _conf['mback_norm']
     opts = {}
     for opt_name in ('z', 'edge', 'e0', 'pre1', 'pre2', 'norm1', 'norm2',
                      'nnorm', 'nvict'):