update(station_factory): imt ue & bs antenna deduplication

[artistrea]

After the previous PRs #214 and #215 , using a different file, with P.619 and using imt.ue.antenna.pattern = OMNI instead of ARRAY. In the first ten drops, the total time taken was 28.6 seconds, 1/3 of which because of antenna creation:

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
   130744    0.210    0.000   11.909    0.000 antenna_factory.py:21(create_antenna)
       10    1.407    0.141   10.252    1.025 station_factory.py:70(generate_imt_base_stations)
    65367    0.709    0.000    7.321    0.000 antenna_s1528.py:27(__init__)

Using this PR speeds up the simulation considerably.

[brunohcfaria]

Please, fix lint errors.