Refactor GpuVector to wrap GpuBuffer

[havahol]

Refactor GpuVector to own GPU memory via GpuBuffer

Summary

Previously, GpuVector duplicated responsibilities already handled by GpuBuffer: allocation, deallocation, size tracking, and parts of host/device transfer.

This PR refactors GpuVector to wrap a GpuBuffer member, reusing GpuBuffer for raw GPU storage while keeping vector-specific operations (cuBLAS, async copies, index-set helpers, dim instead of size) in GpuVector.

The public GpuVector API is unchanged.

Changes

GpuVector refactor

• Replaced T* m_dataOnDevice and int m_numberOfElements with GpuBuffer<T> m_buffer.
• Delegated equivalent functionality to GpuBuffer where possible (constructors, data(), dim(), host/device copies for Dune::BlockVector, etc.).
• GpuBuffer stores size as size_t; cuBLAS calls now use detail::to_int() at the call site instead of storing size as int in GpuVector.
• Added checkedSize() to validate that requested sizes fit in int during construction and resize() (cuBLAS requirement).

GpuBuffer fixes and tests

Expanded test_GpuBuffer coverage, which exposed and fixed two bugs:

• resize: growing a buffer updated m_numberOfElements but did just re-allocate the old capacity.
• copyToHost(BlockVector): size check compared against size() instead of dim() (total scalar count = size() * block_size = dim()).

Pointer attribute helpers

• Renamed is_gpu_pointer.hpp to gpu_pointer_attributes.hpp.
• Added isCPUPointer() to detect whether a pointer refers to host memory (used when validating host pointers passed to GpuBuffer constructors).

Test plan

(Suggested by the LLM that fixed all my markdown encodings - not sure if you usually specify this, or if it is standard anyway)

• Build GPU-enabled opm-simulators
• ctest -R 'TestGpuVector|TestGpuBuffer|test_gpu_pointer_attributes' --output-on-failure
• Run broader gpuistl tests that use GpuVector (e.g. TestGpuJac, TestGpuDILU, test_preconditioner_factory_gpu), or simply all tests.
• Confirm tests still pass after hipify (HIP build)

Next steps

• Move all cudaMemcpy calls into reusable utility functions in gpuistl::detail.
• Centralize pointer-attribute checks (e.g. host vs device) in those helpers so GpuVector, GpuBuffer, and other gpuistl types can share the same logic.
• Extend GpuBuffer with async transfers.