Fix behavior when there's no EDFA DGT

We ship "some" DGT values which effectively mask this, but it's possible
to provide a "trivial" DGT vector such as [0, 0]. When that happens, the
code was failing with a numpy-level warning related to a division by
zero.

The code tried to be ready for this by trying to catch an exception, but
this relied on a particular numpy behavior upon zero division which was
not set up properly. For the added fun, there are two possible cases of
division by zero:

- on a zero tilt, it's a case of `0.0 / np.float64(0.)`, which is
controlled via "invalid",
- on a non-zero tilt, it's a case of `<float> / np.float64(0.)`, which
leads to "divide".

Let's just check for zero instead of wrestling exceptions.

Change-Id: I7a3c0b6b9b85e90a8a7201b7c7a56a5a44c27d69

gnpy/core/elements.py

@@ -824,10 +824,7 @@ class Edfa(_Node):
         targ_slope = -self.tilt_target / (self.params.f_max - self.params.f_min)
 
         # first estimate of DGT scaling
-        try:
-            dgts1 = targ_slope / dgt_slope
-        except ZeroDivisionError:
-            dgts1 = 0
+        dgts1 = targ_slope / dgt_slope if dgt_slope != 0. else 0.
 
         # when simple_opt is true, make 2 attempts to compute gain and
         # the internal voa value. This is currently here to provide direct