Add twts laser

This feature implements a Traveling-wave Thomson-scattering (TWTS) laser pulse by extending the fieldBackground feature of picongpu, which affects only the particle pusher.

- Runtime-test have shown good reproduction of all TWTS E- and B-fields up to float-accuracy at the correct place.
- Adjusted MySimulation.hpp to show the TWTS field to all plugins.
- A new example set "TWTSTest" has been provided.
- The complex number class "complex.hpp" by Richard has been extended.
- Extended interface of componentsConfig.param.
- The calculations in bgrTWTS.hpp require float_64 precision for complex csqrt(), abs() and cexp().

Note: In one particular test run something weird has happened in the middle of the simulation, a while after the sliding window was switched on. It looked like artifacts from absorbers that are propagating inwards. Since the absorbers should not be affected here at all, this might be a race condition?
@ax3l The namespace solution has the advantage that one can have all laser parameters in a central *.param file without glueing these into a class or a functor next to the implementation code (in a single file) . So I actually like your namespace-weirdness, because it keeps the configuration separate from the "real code", while one is completely free to add more models.