FDTD Solver / Free Electron Laser for ALPINE

[manuel5975p]

Open questions

• How to incorporate boundary conditions? They are incompatible with ippls BCType concept
• Should Electrostatic and Electromagnetic Solvers implement the same interface?

Design

[manuel5975p]

Update: Now everything is in the /fel directory I also added a matrix type in Types/Matrix. The lorentz transform classes depend on it.

[aaadelmann]

There are still a few open comments from Sonali to be addressed.

[srikrrish]

I am done with my comments. But it seems this branch may still have some conflicts that need to be resolved.