text: Title: Replacing the obsolete applymap with map
  Body: [DataFrame.applymap](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.applymap.html) 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)
  ```