SparsityPattern::Iterator - add a constructor for initalizing an iterator from an...

SparsityPattern::Iterator  - add a constructor for initalizing an iterator from an accessor

When trying to increment a sparsity pattern iterator:

  auto iterator = sparsity_pattern.begin();
  iterator++;

The corresponding function in LinearIndexIterator:

  template <class DerivedIterator, class AccessorType>
  inline DerivedIterator
  LinearIndexIterator<DerivedIterator, AccessorType>::operator++(int)
  {
    const DerivedIterator copy(this->accessor);
    operator+=(1);
    return copy;
  }

creates a copy that is initialized by the accessor. Thus, we need a
constructor that can create an interator from an accessor.