Add ability to get an array from a Field for a particular type

Previously, whenever you got an array from a Field object from a call to an ApplyPolicy, you would get back a VariantArrayHandle that allows you to cast to multiple types. To use that, you then have to cast it to multiple different types and multiple different storage.

Often, this is what you want. If you are operating on a field, then you want to cast to the native type. But there are also cases where you know a specific type you want. For example, if you are operating on two fields, it makes sense to find the exact type for the first field and then cast the second field to that type if necessary rather than pointlessly unroll templates for the cross of every possible combination. Also, we are not unrolling for different storage types or attempting to create a virtual array. Instead, we are using an ArrayHandleMultiplexer so that you only have to compile for this array once.

This is done through a new version of ApplyPolicy. This version takes a type of the array as its first template argument, which must be specified.

This requires having a list of potential storage to try. It will use that to construct an ArrayHandleMultiplexer containing all potential types. This list of storages comes from the policy. A StorageList item was added to the policy.

Types are automatically converted. So if you ask for a vtkm::Float64 and field contains a vtkm::Float32, it will the array wrapped in an ArrayHandleCast to give the expected type.

Here is an example where you are doing an operation on a field and coordinate system. The superclass finds the correct type of the field. Your result is just going to follow the type of the field.

template <typename T, typename StorageType, typename DerivedPolicy>
inline VTKM_CONT vtkm::cont::DataSet CrossProduct::DoExecute(
  const vtkm::cont::DataSet& inDataSet,
  const vtkm::cont::ArrayHandle<T, StorageType>& field,
  const vtkm::filter::FieldMetadata& fieldMetadata,
  vtkm::filter::PolicyBase<DerivedPolicy> policy)
{
  vtkm::cont::CoordinateSystem coords = inDataSet.GetCoordianteSystem();
  auto coordsArray = vtkm::filter::ApplyPolicy<T>(coords, policy);