Cast cell types as integer when parsing xml

Since some files can give non uint8 type cell types, we explicitly cast
everything to the right type instead of expecting it to be given
correctly. This helps parse files generated by meshio (See issue #21)

src/xml.rs

@@ -1529,6 +1529,23 @@ impl Cells {
         }
     }
 
+    /// Decodes a data array for types.
+    ///
+    /// These can be specified as u8 or some other integer type.
+    /// This logic is encapsulated in this function.
+    fn get_type_codes(
+        buf: model::IOBuffer,
+    ) -> std::result::Result<Vec<model::CellType>, ValidationError> {
+        use num_traits::FromPrimitive;
+        let type_codes = buf
+            .cast_into::<u8>()
+            .ok_or_else(|| ValidationError::InvalidDataFormat)?;
+        type_codes
+            .into_iter()
+            .map(|x| model::CellType::from_u8(x).ok_or_else(|| ValidationError::InvalidCellType(x)))
+            .collect()
+    }
+
     /// Given the expected number of elements and an optional appended data,
     /// converts this `Topo` struct into a `mode::VertexNumbers` type.
     pub fn into_model_cells(
@@ -1537,15 +1554,7 @@ impl Cells {
         appended: Option<&AppendedData>,
         ei: EncodingInfo,
     ) -> std::result::Result<model::Cells, ValidationError> {
-        use num_traits::FromPrimitive;
-
-        let type_codes: Option<Vec<u8>> = self.types.into_io_buffer(l, appended, ei)?.into();
-        let type_codes = type_codes.ok_or_else(|| ValidationError::InvalidDataFormat)?;
-        let types: std::result::Result<Vec<model::CellType>, ValidationError> = type_codes
-            .into_iter()
-            .map(|x| model::CellType::from_u8(x).ok_or_else(|| ValidationError::InvalidCellType(x)))
-            .collect();
-        let types = types?;
+        let types = Self::get_type_codes(self.types.into_io_buffer(l, appended, ei)?)?;
 
         let offsets: Option<Vec<u64>> = self.offsets.into_io_buffer(l, appended, ei)?.cast_into();
         let offsets = offsets.ok_or_else(|| ValidationError::InvalidDataFormat)?;