Pandas multi-dimensional indexing FutureWarning for `epra.analyze_all_variations`

[nikosavola]

On latest pandas versions, calling epra.analyze_all_variations results in the following warnings:

c:\pyepr\pyEPR\core_quantum_analysis.py:712: FutureWarning: Support for multi-dimensional indexing (e.g. `obj[:, None]`) is deprecated and will be removed in a future version.  Convert to a numpy array before indexing instead.
  result['Q_coupling'] = self.Qm_coupling[variation][self.Qm_coupling[variation].columns[junctions]][modes]