feature, add NT2 support for HDF5 I/O

[steven-varga]

Add h5cpp I/O integration for NT2 numerical template toolkit.\n\nTask:\n- Vendor NT2 headers into thirdparty/nt2/\n- Create h5cpp/H5Mnt2.hpp\n- Add example examples/linalg/nt2.cpp\n\nAcceptance Criteria:\n- [ ] NT2 table/array writes to HDF5 and reads back correctly