Fix deprecation warnings
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.