Refactor `DataFrame.append` to `pd.concat`

[nikosavola]

This resolves the following warnings emerging during EPR analysis

pyepr\pyEPR\core_distributed_analysis.py:1307: FutureWarning: The series.append method is deprecated and will be removed from pandas in a future version. Use pandas.concat instead.
  sol = sol.append(self.get_Qdielectric(

Closes #136

[zlatko-minev]

Great, thank you!

[zlatko-minev]

There is a conflict

<<<<<<< 136-pandas-series-append-deprecation
                if self.pinfo.dissipative['resistive_surfaces']:
                    if self.pinfo.dissipative['resistive_surfaces'] == 'all':
                        sol = pd.concat([sol, self.get_Qsurface_all(mode, variation, self.U_E)])
=======
                if self.pinfo.dissipative['dielectric_surfaces']:
                    if self.pinfo.dissipative['dielectric_surfaces'] == 'all':
                        sol = sol.append(
                            self.get_Qsurface_all(mode, variation, self.U_E))
>>>>>>> master

Could you resolve?

[zlatko-minev]

Thank you!