Deprecated functions of emcee used

[BerndDoser]

Warning during pytest of fit_background:

  /usr/local/lib/python3.8/dist-packages/emcee/utils.py:23: DeprecationWarning: 'lnprobability' is deprecated. Use 'get_log_prob()' instead.
    deprecation_warning(msg)

tests/test_background_fit.py: 16 warnings
  /usr/local/lib/python3.8/dist-packages/emcee/utils.py:23: DeprecationWarning: 'chain' is deprecated. Use 'get_chain()' instead.
    deprecation_warning(msg)

tests/test_background_fit.py::test_background_fit_001296068
  /usr/local/lib/python3.8/dist-packages/emcee/utils.py:23: DeprecationWarning: 'flatchain' is deprecated. Use 'get_chain(flat=True)' instead.
    deprecation_warning(msg)

Simple interchange with proposed new function gives index error.

[BerndDoser]

Old API (version 2.2.1): https://emcee.readthedocs.io/en/v2.2.1/api/#emcee.EnsembleSampler.lnprobability
New API (version 3.1.2): https://emcee.readthedocs.io/en/v3.1.2/user/sampler/#emcee.EnsembleSampler.get_log_prob

In the deprecated warning of emcee a index change is applied: https://github.com/dfm/emcee/blob/9b3a047976a3723a179456d63e60f046bab423f5/src/emcee/ensemble.py#L557