Use nanosleep as waiter in PoissonRecon process

This pull request intends to fix same issue than cloudcompare/trunk@f686522a0da7a2bc431ce4b49d4162e2d5a16bb8 but for Poisson Reconstruction plugin. it seems that sleep and usleep are deprecated in favor of nanosleep. See http://man7.org/linux/man-pages/man3/usleep.3.html for further information.