Replacing the obsolete applymap with map

[quant12345]

DataFrame.applymap has been deprecated.

I'd like to do a PR to fix this. What does the community think?

Warning:

darts/tests/test_timeseries_multivariate.py::TestTimeSeries::test_map

  /darts/darts/tests/test_timeseries.py:1449: FutureWarning: DataFrame.applymap has been deprecated. Use DataFrame.map instead.
    df_0[["0"]] = df_0[["0"]].applymap(fn)

darts/tests/test_timeseries.py::TestTimeSeries::test_map
darts/tests/test_timeseries_multivariate.py::TestTimeSeries::test_map
  /darts/darts/tests/test_timeseries.py:1450: FutureWarning: DataFrame.applymap has been deprecated. Use DataFrame.map instead.
    df_2[["2"]] = df_2[["2"]].applymap(fn)

darts/tests/test_timeseries.py::TestTimeSeries::test_map
darts/tests/test_timeseries_multivariate.py::TestTimeSeries::test_map
  /darts/darts/tests/test_timeseries.py:1451: FutureWarning: DataFrame.applymap has been deprecated. Use DataFrame.map instead.
    df_01[["0", "1"]] = df_01[["0", "1"]].applymap(fn)

darts/tests/test_timeseries.py::TestTimeSeries::test_map
darts/tests/test_timeseries_multivariate.py::TestTimeSeries::test_map
  /darts/darts/tests/test_timeseries.py:1452: FutureWarning: DataFrame.applymap has been deprecated. Use DataFrame.map instead.
    df_012 = df_012.applymap(fn)

[madtoinou]

Hi @quant12345,

Sure, you can go ahead :)

[quant12345]

Hi @madtoinou, contribution says that need an issue first, so I asked)

[dennisbader]

Fixed in #2436