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?
I can add a paper reference (I will), but you will not find these equations, because these time-domain equations are simply derived new and found nowhere else, except in my notebooks.
The code is perfectly readable, it is just long. And since long equations are difficult to implement, the primary mission here is is not to satisfy some specific definition of readability, but the ability to validate the code. What you really want to do is check the programs and methods that created this code.