Title: [Patch] Use MetaImage Name metadata as the scalar-array name

I'd like to propose a small change to `vtkMetaImageReader` for `.mha` and `.mhd` files. MetaIO already parses the optional `Name` header field, but VTK currently assigns the point-data scalar array the hardcoded name `MetaImage`.

The attached patch uses the existing `Name` value as the scalar-array name when it is nonempty. For example, `Name = Temperature` produces a scalar array named `Temperature`. Files with no name, an empty name, or a whitespace-only name retain the existing `MetaImage` default. As with other MetaImage header metadata, `Name` must appear before `ElementDataFile`.

My motivation is loading several MetaImage volumes in ParaView with distinct array names, allowing separate color and opacity transfer functions. The change is confined to VTK's reader; it introduces no new metadata fields or file-format changes. The name is assigned after the successful data read so it reflects the metadata read with the voxel data.

The patch includes documentation and a synthetic regression test covering both file extensions, named and unnamed inputs, empty names, reader reuse, repeated updates, and voxel integrity. The regression passes with the patch and fails on named inputs with the original reader. The existing MetaIO round-trip test also passed on synthetic data. ParaView integration has not been tested.

Feedback on this approach would be welcome.

---

Patch base: VTK commit `5cf9751543ecd5370c65ffeab656ce09bb8a2c9d`.

Apply from the VTK source root:

```sh
git apply --check vtk-metaimage-reader-name.patch
git apply vtk-metaimage-reader-name.patch
```