To get the average, we (of course) divide the sum by the amount of
values, which is returned from valuesIn.GetNumberOfComponents(). To do
this, we need to cast the number of components (returned as a
vtkm::IdComponent) to a FieldType. This is a little more complex than it
first seems because FieldType might be a Vec type. If you just try a
static_cast<FieldType>(), it will use the constructor to FieldType which
might be a Vec constructor expecting the type of the component. This in
turn could cause a warning because the vtkm::IdComponent is implicitly
converted to the Vec's component type.

Get around this problem by first casting to the component type of the
field and then constructing a field value from that.