Implement Sum.findrecur (with Sister Celine's algorithm)

fix #5511

- [ ] better name?
- [ ] Not sure if this should be a Sum method.  An alternative: some helper function.
> For example?  If something "goes wrong" - the current code should
> raise ValueError (L: 483).

I did not try nor find an example where something goes wrong.

In general I like for loops with fixed maximal range better
than while loops.

If you think it's safe enough then this is ok.

> Though, from eq. 4.4.3 of AeqB: it seems to be I* >= J*.  So, the
> current code could stop symmetical increasing of I and J at some
> point. Not sure if it worth for code complication.

Well, I think it depends on the relative size of J* and and I*.
There could be a gain. For the moment we might keep it symmetrical.