Const-correctness: `sidre::View::getData<T>` should be marked `const`

This function returns a copy of the data in the sidre view and does not modify the data, so it should be marked `const`

I tried to call `getData()` from a `const View` and could not since the `const` was missing.

See: https://github.com/LLNL/axom/blob/9fb22cd277211d8a2092558c915aa574b709a722/src/axom/sidre/core/View.hpp#L929-L940