Add birch_murnaghan to results.eos fit functions
Fixes issue with TypeError: The value birch_murnaghan is not an enum value for quantity nomad.datamodel.results.EnergyVolumeCurve.type (nomad github issue 37)
Fixes issue with TypeError: The value birch_murnaghan is not an enum value for quantity nomad.datamodel.results.EnergyVolumeCurve.type (nomad github issue 37)