Introduce PDL accumulation and penalty calculation

This fixes #421

As a first step PDL is specified in the eqpt library for ROADMs only.
In a later step, PDL (as well as PMD) should be specified also for amps
and possibly for fibers. PDL values from the OpenROADM MSA for ROADMs
are included in the corresponding eqpt files.

The acculumation rule for PDL is the same as for PMD as shown in:

"The statistics of polarization-dependent loss in optical communication
systems", A. Mecozzi and M. Shtaif, IEEE Photon. Technol. Lett., vol.
14, pp. 313-315, Mar 2002.

PDL penalty is specified and calculated in the same way as for CD and
PMD, i.e. linear interpolation between impairment_value/penalty_value
pairs. This patch includes penalty specification for OpenROADM trx
modes according to the MSA.

Signed-off-by: Jonas Mårtensson <jonas.martensson@ri.se>
Change-Id: Ib0ab383bcaee7d7523ffc3fa9a949d76c8c86ff7

gnpy/tools/json_io.py

@@ -94,6 +94,7 @@ class Roadm(_JsonThing):
         'target_pch_out_db': -17,
         'add_drop_osnr': 100,
         'pmd': 0,
+        'pdl': 0,
         'restrictions': {
             'preamp_variety_list': [],
             'booster_variety_list': []