NameProxy const ness

[romainfrancois]

This is bad:

    inline NameProxy operator[]( const std::string& name ) const {
        return NameProxy( const_cast<Vector_Impl&>(*this), name ) ;
    }
    inline NameProxy operator()( const std::string& name ) const {
        return NameProxy( const_cast<Vector_Impl&>(*this), name ) ;
    }

We should have a const_NameProxy class and avoid doing const_cast

[github-actions]

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