Support propagation of single channel

It always seemed like a strange restriction to not allow this.

Change-Id: Ice3ed3ecc08f42b6ef8b74d4a6bc3b1794ff078a
Signed-off-by: Jonas Mårtensson <jonas.martensson@ri.se>

gnpy/core/elements.py

@@ -725,6 +725,9 @@ class Edfa(Node):
         """
 
         # TODO|jla: check what param should be used (currently length(dgt))
+        if len(self.interpol_dgt) == 1:
+            return array([self.effective_gain])
+
         nb_channel = arange(len(self.interpol_dgt))
 
         # TODO|jla: find a way to use these or lose them. Primarily we should have