Sampling without replacement in jDE

[neduard]

Inspired from #95 (comment)
When taking population samples in jDE, if the population size (NP) is small enough, we apply a sampling without replacement algorithm, otherwise we revert to previous behaviour.

Testing was done by make tests and a custom test bench:
https://github.com/neduard/pagmo/blob/development/my_tutorials/myjdedev.cpp

This patch addresses issue #95

[neduard]

Currently we include everything in the evolve() method. Would it be a good idea to separate the sampling algorithm (both for small and large NP) into a separate function?

[krzysztof]

@neduard Putting it into an external function is not necessary as it's so far the only place we use sampling wo replacement. Otherwise it would go in pagmo::util. Patch is fine as it is for now.

[landscape-bot]

Code quality remained the same when pulling 255ba78 on neduard:issue95_sampling_without_replacement_jde into 6ec99bc on esa:master.