feature, add GMTL support for HDF5 I/O

[steven-varga]

Add h5cpp I/O integration for GMTL Generic Graphics Toolkit matrices/vectors.\n\nTask:\n- Vendor GMTL headers into thirdparty/gmtl/\n- Create h5cpp/H5Mgmtl.hpp\n- Add example examples/linalg/gmtl.cpp\n\nAcceptance Criteria:\n- [ ] GMTL Matrix/Vec writes to HDF5 and reads back correctly