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.
On Mon, Oct 20, 2014 at 03:51:20PM -0700, raoulb wrote:
>    At least one could break the loop there. This would avoid run-away loops
>    in case anything within the body goes wrong.

For example?  If something "goes wrong" - the current code should raise
ValueError (L: 483).

>    The global structure is: try
>    with this i, j and if it does not succeed then increase the values. It
>    should be possible to stop once we reach the upper bound anyway.

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.