Added ability to set material otf file to None
Added ability to set material otf file to None

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