Synchronization at the end of DASH algorithms

[devreal]

While working on #374 I realized that some algorithms in DASH synchronize units at the end (e.g., dash::for_each) and some don't (e.g., dash::generate). This should be unified.

@fuchsto I assume this is covered by Execution Policies (#104)? If so, this is just to track progress on this issue.

[fuchsto]

Yes, exactly, but it's sensible to track this as a separate issue.