tests: frequency to wavelength

I decided to keep it around because I know that some people would like
to see those nanometers. Let's make sure it works.

Change-Id: Ib279cc8380a77f478da7a2bbc1e045a718446404

gnpy/core/utils.py

@@ -131,6 +131,11 @@ freq2wavelength = constants.nu2lambda
 
 def freq2wavelength(value):
     """ Converts frequency units to wavelength units.
+
+    >>> round(freq2wavelength(191.35e12) * 1e9, 3)
+    1566.723
+    >>> round(freq2wavelength(196.1e12) * 1e9, 3)
+    1528.773
     """
     return constants.c / value