Make Exodus writer handle a mesh block with mixed types of elements

[YaqiWang]

Exodus format requires each mesh block to have only one type of elements, which is not a requirement for libMesh data structure, nor possibly some other mesh formats. In cases that we have a mixed types of elements in a mesh and we use Exodus writer, we will get an error message. We possibly can improve the writer so that it can automatically split the mesh block based on the element types. If a mesh block only contains one element type, it will be a no-op, otherwise, it appends the block name with _[ElemType], like _TRI3. In this way, the writer can still work properly without erroring out. Tag @roystgnr