Fix 'atom/b-cm' density type in PyAPI
Fix 'atom/b-cm' density type in PyAPI

smharper/openmc,liangjg/openmc,liangjg/openmc,paulromano/openmc,walshjon/openmc,smharper/openmc,wbinventor/openmc,walshjon/openmc,amandalund/openmc,liangjg/openmc,smharper/openmc,shikhar413/openmc,wbinventor/openmc,mit-crpg/openmc,amandalund/openmc,mit-crpg/openmc,shikhar413/openmc,bhermanmit/openmc,amandalund/openmc,samuelshaner/openmc,wbinventor/openmc,bhermanmit/openmc,shikhar413/openmc,paulromano/openmc,johnnyliu27/openmc,mit-crpg/openmc,wbinventor/openmc,smharper/openmc,walshjon/openmc,johnnyliu27/openmc,mjlong/openmc,mjlong/openmc,johnnyliu27/openmc,shikhar413/openmc,samuelshaner/openmc,kellyrowland/openmc,amandalund/openmc,paulromano/openmc,mit-crpg/openmc,liangjg/openmc,johnnyliu27/openmc,paulromano/openmc,kellyrowland/openmc,samuelshaner/openmc,walshjon/openmc,samuelshaner/openmc