Rework PSO

[HeleNoir]

PR #81 presented some minor issues with PSO. These should be handled at some point, presumably when other swarm algorithms are added.

• [x] factor variables for velocity update
• [ ] allow for local best and global best orientation
• [x] move PSO operations to make them generally applicable

[HeleNoir]

Item 1 and 3 are handled in #112 . Item 2 has now its own issue #113.