`Jackknife` estimator is very slow

[Datseris]

I've noticed that Jacknife is very slow and could benefit by many improvements:

• better estimation of the missing elements of x by using deleteat! and insertat!
• parallelization over threads (this is actually trivial to do)
• having an optional progress bar (new field to Jackknife)
• Warning users that it takes miuch more than a typical entropy estimat as it computes at least as many entropies as length(x).