Do not use confusing default values

This looks like the root cause of bug #243, the default values suggested
that this function works in THz.

These defaults are not used anywhere, so let's get rid of something
which adds no value and actively confuses people.

gnpy/core/utils.py

@@ -73,9 +73,8 @@ def c():
     return constants.c
 
 
-def itufl(length, startf=191.35, stopf=196.10):
-    """Creates an array of frequencies whose default range is
-    191.35-196.10 THz
+def itufl(length, startf, stopf):
+    """Create an array of frequencies
 
     :param length: number of elements
     :param starf: Start frequency in THz