Change sign convention for amplifier tilt

As pointed out in GitHub issue #390, the normal convention for the sign
of amplifier tilt is to define it with regard to wavelength, i.e.
negative tilt means lower gain for longer wavelengths (lower
frequencies). Currently GNPy uses the opposite convention, which this
patch proposes to change.

Signed-off-by: Jonas Mårtensson <jonas.martensson@ri.se>
Change-Id: I8f7829a3b0b0b710f7da013c525630a60b22a2b5

gnpy/core/elements.py

@@ -814,7 +814,7 @@ def _gain_profile(self, pin, err_tolerance=1.0e-11, simple_opt=True):
         dgt_slope = p[0]
 
         # Calculate the target slope
-        targ_slope = self.tilt_target / (self.params.f_max - self.params.f_min)
+        targ_slope = -self.tilt_target / (self.params.f_max - self.params.f_min)
 
         # first estimate of DGT scaling
         try: