SparseCompressedBase::InnerIterator has operator++ but no operator += or operator +

There are cases where one may want to advance the operator by N elements. Currently, the only way to do it is with a for loop such as :

for (i = 0; i < N; ++i,++iterator) {}

An addition operator is easy to define within the iterator :

inline InnerIterator& operator+=( _Index i ) { m_id += i; return *this; }
inline InnerIterator operator+( _Index i ) 
{
    InnerIterator result = *this;
    result += i;
    return result;
}

Similarly, a substraction operator can be added to ReverseIterator

inline ReverseInnerIterator& operator-=( _Index i ) { m_id -= i; return *this; }
inline ReverseInnerIterator operator-( _Index i ) 
{
    ReverseInnerIterator result = *this;
    result -= i;
    return result;
}