Add time stepper pt storage to array interpolator

(other interpolator has it)
Unify implicit/semi implicit  micromag elements

Using new MagnetostaticsCalculator class.

Should simplify mesh construction a lot.

Allows other ways of calculating h_ms
Unify implicit/semi-implicit mesh construction
Add access to dim in ArrayInterpolator
Use vectors in magnetostatic field functions
Add function for magnetostatic field due to a sphere