BUG: pandas future warnings

[aburrell]

Description

There are several future warnings to address

To Reproduce this bug:

  /home/runner/work/pysatMadrigal/pysatMadrigal/pysatMadrigal/instruments/methods/general.py:1363: FutureWarning: The behavior of array concatenation with empty entries is deprecated. In a future version, this will no longer exclude empty items when determining the result dtype. To retain the old behavior, exclude the empty entries before the concat operation.
    out = pds.concat(out_series).sort_index()

pysatMadrigal/tests/test_instruments.py: 18 warnings
  /opt/hostedtoolcache/Python/3.10.13/x64/lib/python3.10/site-packages/pysat/_instrument.py:2205: FutureWarning: 'S' is deprecated and will be removed in a future version, please use 's' instead.
    file_inc = pds.tseries.frequencies.to_offset(file_freq)

pysatMadrigal/tests/test_instruments.py::TestInstruments::test_download[inst_dict13]
  /home/runner/work/pysatMadrigal/pysatMadrigal/pysatMadrigal/instruments/dmsp_ssj.py:270: FutureWarning: 'AS-JAN' is deprecated and will be removed in a future version, please use 'YS-JAN' instead.
    if date_array.freq not in ['AS-JAN', 'YS', 'AS']:

pysatMadrigal/tests/test_instruments.py::TestInstruments::test_download[inst_dict13]
  /home/runner/work/pysatMadrigal/pysatMadrigal/pysatMadrigal/instruments/dmsp_ssj.py:270: FutureWarning: 'AS' is deprecated and will be removed in a future version, please use 'YS' instead.
    if date_array.freq not in ['AS-JAN', 'YS', 'AS']:

Test configuration

• OS: All
• Version: All
• Other details about your setup that could be relevant: pandas v2.2.1

[aburrell]

Also:

  You are setting values through chained assignment. Currently this works in certain cases, but when using Copy-on-Write (which will become the default behaviour in pandas 3.0) this will never work to update the original DataFrame or Series, because the intermediate object on which we are setting values will behave as a copy.
  A typical example is when you are setting values in a column of a DataFrame, like:
  
  df["col"][row_indexer] = value
  
  Use `df.loc[row_indexer, "col"] = values` instead, to perform the assignment in a single step and ensure this keeps updating the original `df`.
  
  See the caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy
  
    self[dvar][self[dvar] == -32766] = np.nan