Feat: add offset power option for transceivers

Offset power is used for equalization purpose to correct for the
generic equalization target set in ROADM for this particular transceiver.
This is usefull to handle exception to the general equalization rule.
For example in the case of constant power equalization, the user might
want to apply particular power offsets unrelated to slot width or baudrate.
or in constant PSW, the user might want to have a given mode equalized for
a different value than the one computed based on the request bandwidth.

For example consider that a transceiver mode is meant to be equalized with
75 GHz whatever the spacing specified in request. then the user may specify
2 flavours depending on used spacing:

  service 1 : mode 3, spacing 75GHz
  service 2 : mode 4, spacing 87.5Ghz
avec
  {
    "format": "mode 3",
    "baud_rate": 64e9,
    "OSNR": 18,
    "bit_rate": 200e9,
    "roll_off": 0.15,
    "tx_osnr": 40,
    "min_spacing": 75e9,
    "cost": 1
  }

  {
    "format": "mode 4",
    "baud_rate": 64e9,
    "OSNR": 18,
    "bit_rate": 200e9,
    "roll_off": 0.15,
    "tx_osnr": 40,
    "min_spacing": 87.5e9,
    "equalization_offset_db": -0.67,
    "cost": 1
  }

then the same target power would be considered for mode3 and mode4
despite using a different slot width

Signed-off-by: EstherLerouzic <esther.lerouzic@orange.com>
Change-Id: I437f75c42f257b88b24207260ef9ec9b1ab7066e

gnpy/tools/cli_examples.py

@@ -331,7 +331,6 @@ def path_requests_run(args=None):
     # TODO power density: db2linp(ower_dbm": 0)/power_dbm": 0 * nb channels as defined by
     # spacing, f_min and f_max
     p_db = equipment['SI']['default'].power_dbm
-
     p_total_db = p_db + lin2db(automatic_nch(equipment['SI']['default'].f_min,
                                              equipment['SI']['default'].f_max, equipment['SI']['default'].spacing))
     try: