Fix calculation of gain for first Edfa after Roadm
Fix calculation of gain for first Edfa after Roadm

See issue reported in #360

In autodesign, currently the calculation of gain for the first Edfa
after a Roadm is incorrect when the reference channel power is
different from 0 dBm. The bug is somewhat hidden by the fact that the
gain is anyway updated during propagation in power mode, taking the
reference channel power into account. But the gain reported by the
to_json function of Edfa elements before propagation will be wrong.
And more seriously, the incorrect gain will impact the Edfa selection
in autodesign.

Change-Id: I004de102832c3a0786435e21e71b0444d8901604
Signed-off-by: Jonas Mårtensson <9c18489050b439bcd0a9df0d47735ceb99443dbb@ri.se>

Telecominfraproject/gnpy