feature, add xtensor-blas support for HDF5 I/O

[steven-varga]

Add h5cpp I/O integration for xtensor-blas BLAS bindings.\n\nTask:\n- Vendor xtensor-blas headers into thirdparty/xtensor-blas/\n- Create h5cpp/H5Mxtensorblas.hpp (may reuse xtensor base types)\n- Add example examples/linalg/xtensor-blas.cpp\n\nBlocked by: #146 (xtensor base support)\n\nAcceptance Criteria:\n- [ ] xtensor-blas linalg expressions write/read correctly