Add modality to schema for sets and elements

[shadowdoc]

Will be optional. Pasting the original modality specification from legacy schema below

modality =
    element modality {
        element system { "RadLex" },
        ( element code {"CT"},
           element name { "Computed Tomography"},
           element url { "http://radlex.org/RID/RID10321" } ) ?,
        ( element code { "FL" },
          element name { "Fluoroscopy" },
          element url { "http://radlex.org/RID/RID10361" } ) ?,
        ( element code { "MR" },
          element name { "Magnetic Resonance Imaging" },
          element url { "http://radlex.org/RID/RID10312" } ) ?,
        ( element code { "NM" },
          element name { "Nuclear Medicine" },
          element url { "http://radlex.org/RID/RID10330" } ) ?,
        ( element code { "PET" },
          element name { "Positron Emission Tomography" },
          element url { "http://radlex.org/RID/RID10337" } ) ?,
        ( element code { "US" },
          element name { "Ultrasound" },
          element url { "http://radlex.org/RID/RID10326" } ) ?,
        ( element code { "XR" },
          element name { "Projection Radiography" },
          element url { "http://radlex.org/RID/RID10345" } ) ?
     }

[shadowdoc]

Duplicate with newer #34