obi import: small efficiency improvement when dealing with NA values

python/obitools3/commands/import.pyx

@@ -79,6 +79,7 @@ def run(config):
     cdef   list        old_elements_names
     cdef   list        new_elements_names
     cdef   ProgressBar pb
+    cdef   bytes       NA_value
     global             obi_errno
         
     
@@ -102,6 +103,8 @@ def run(config):
     
     iseq = input[1]
     
+    NA_value = config['obi']['nastring']
+    
     get_quality = False
     NUC_SEQS_view = False
     if isinstance(output[1], View) :
@@ -143,7 +146,7 @@ def run(config):
                 value = seq[tag]
                                 
                 # Check NA value
-                if value == config['obi']['nastring'] :
+                if value == NA_value :
                     value = None
                  
                 if tag not in dcols :