[core][mback_norm] fix configuration read from mback_norm

4 jobs for larch_usability in 4 minutes and 35 seconds
Status Name Job ID Coverage
  Test
passed test:python3.5-stretch-pyqt5_larch #45470

00:03:55

passed test:python3.5-stretch-pyqt5_pymca #45469

00:04:24

passed test:python3.5-stretch-pyqt5_pymca_larch #45471

00:04:35

passed test:test-xas-tutorials_pymca #45472

00:03:14