STYLE: Use `const auto` for variables made by New(), in elxTransformBase
STYLE: Use `const auto` for variables made by New(), in elxTransformBase

- Increased const-correctness. Note that the added `const` keywords are "shallow": they just prevents resetting the pointers, initialized by `New()`.
- Reduced redundancy, by avoiding to mention the type of each variable made by `New()` twice.

SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix,SuperElastix/elastix