ENH: Clarify and document leash parameter in simpdiv

[tturocy]

There is a leash parameter in simpdiv which is designed to rein in very long paths. This was something RDM implemented in the early 1990s based on some experience with pathologically long paths. Adding this constraint trades off guaranteed global convergence of the algorithm against very long running times.

I would not suggest removing the parameter, but having it default to being turned on (but large), as it does now, is probably not the right way. This should be reviewed, with the default being no constraint, and then documented properly.