Section find does not include self in result set

[jgrewe]

fixes #690

[achilleas-k]

LOOKS AMAZING TO ME

[gicmo]

While we are there, we could make SectionCont be like:

struct SectionCont {
    SectionCont(const Section &s, size_t d = 0)
    : entity(s), depth(d)
    {}

    const Section &entity;
    size_t depth;
};

I think that should work. Could also replace it with std::tuple, though that might make it less readable.