Fix use of enrichment with Material.add_elements_from_formula
Fix use of enrichment with Material.add_elements_from_formula

walshjon/openmc,shikhar413/openmc,amandalund/openmc,shikhar413/openmc,liangjg/openmc,smharper/openmc,shikhar413/openmc,smharper/openmc,shikhar413/openmc,walshjon/openmc,liangjg/openmc,paulromano/openmc,liangjg/openmc,mit-crpg/openmc,mit-crpg/openmc,amandalund/openmc,walshjon/openmc,paulromano/openmc,walshjon/openmc,mit-crpg/openmc,smharper/openmc,smharper/openmc,paulromano/openmc,paulromano/openmc,mit-crpg/openmc,amandalund/openmc,liangjg/openmc,amandalund/openmc