There was an error that caused deprecation warnings in VTK-m to be
suppressed, which meant that many uses of deprecated features went
unnoticed. This fixes those deprecation warnings.

The majority of the warnings were caused by the use of the deprecated
`Cast`, `CopyTo`, and `ResetTypes` methods of `UnknownArrayHandle` (or
`VariantArrayHandle`). Both `Cast` and `CopyTo` have been subsumed by
`AsArrayHandle` (to make the functionality more clear). `ResetTypes` now
requires a second template argument to define the storage types to try.

Also fixed some issues with `SerializableField` being deprecated.
This class is no longer necessary because `Field` can now be directly
serialized.