Do not use continue stuff. Instead precompute the quantities.

Feature/memory fences

TODOS
* [x] General infrastructure to issue barriers
* [x] Actually implement the barrier in `DuneTarget`
* [ ] Identify the spots in sum factorization where barriers are necessary

This fixes #46 

See merge request !55

Feature/sumfact poisson

See merge request !59

Implement custom error classes and use them

This fixes #45 

See merge request !56

Feature/sumfact mass matrix

See merge request !51