Corrected the LeapFrog algorithm.

* E and B field calculations are now in the right order.
* First all the new E components are calculated only then the B calculation starts. This is the correct LeapFrog scheme.
* The dependence on the old fields was removed.
Merge pull request #51 from kstrel/fieldsolver-corrections

Corrected the SimpleSolver algorithm.

E and B field calculations are now in the right order.
First all the new E components are calculated only then the B calculation starts. This is the correct LeapFrog scheme.
The dependence on the old fields was removed.
This dramatically improves the stability of the whole simulation.