Update settings.py: add flat as a valid ref/flat titles

nxtomomill/settings.py

@@ -89,7 +89,7 @@ class Tomo:
         DARK_TITLES = ("dark images", "dark")
         """if a scan starts by one of those string then is considered as
         dark scan"""
-        REF_TITLES = ("reference images", "ref", "refend")
+        REF_TITLES = ("flat", "reference images", "ref", "refend")
         """if a scan starts by one of those string then is considered as
         reference scan"""
         PROJ_TITLES = ("projections", "ascan rot 0 ", "ascan diffrz 0 180 1600 0.1")