remove use of deprecated register storage class

[minrk]

deprecated in C++11, removed in C++17, causing failures to compile with recent compilers:

In file included from AnalyticalJacobian.cpp:1:
In file included from /Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/AnalyticalJacobian.h:6:
In file included from /Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/Jacobian.h:6:
In file included from /Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/Vec.h:6:
/Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/CMRVec.h:48:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int index=base_,end=base_+dim_;
    ^~~~~~~~~
/Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/CMRVec.h:48:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int index=base_,end=base_+dim_;
    ^~~~~~~~~
/Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/CMRVec.h:61:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int index=base_,end=base_+dim_;
    ^~~~~~~~~
/Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/CMRVec.h:61:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int index=base_,end=base_+dim_;
    ^~~~~~~~~
/Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/CMRVec.h:74:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int index=base_,end=base_+dim_;
    ^~~~~~~~~
/Users/runner/miniforge3/conda-bld/daetk_1696641573451/work/CMRVec.h:74:5: error: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    register int index=base_,end=base_+dim_;
    ^~~~~~~~~

It's possible to also keep it gated on pre-c++17 via a macro, but it was deprecated because it ~never does what was requested, so I suspect removing it is the right thing to do.