Add CoordinateSequenceIterator

[dbaston]

This adds an iterator to CoordinateSequence so that class can be used with STL algortithms

std::sort(seq.begin(), seq.end());

and range for loops,

    for (auto& coord : seq_) {
        coord.y = 2*coord.x;
    }

    // Use const iterator to check values
    std::size_t i = 0;
    for (const auto& coord : seq_) {
        ensure_equals(coord.x, static_cast<double>(i));
        ensure_equals(coord.y, 2*coord.x);
        i++;
    }

This does not reduce the overhead of virtual Coordinate access; that requires #674 .