Describe the bug

Following up on issue #2857:
bluemira.codes.process._solver.get_species_data is the only piece of code where temperature is given in units of eV is instead of keV or K.

Expected behaviour

get_species_data returns t_ref in keV instead of eV.

Suggested fix

t_ref_keV = raw_uc(np.array(next(t_ref).data, dtype=float), "eV", "keV")
return t_ref_keV, np.array(next(lz_ref).data, dtype=float), np.array(next(z_av_ref).data, dtype=float)

Additional context

This change would be API-breaking, thus must be approved by all users before merging into the develop branch. @DarioV86