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?
This is non-trivial and not done below a couple of days. Note that I did already many tests on this. The general chain of tests is as follows:

1.) Analytical TWTS in frequency model in the frequency domain with numerical integration to obtain E and B-fields in the time domain.
2.) Analytical TWTS model in the time domain based on some flavour of the slowly varying wave-approximation for E- and B-fields. The resulting large expressions were checked against 1. using Mathematica.
3.) The equations in 2. were programatically transformed into PIConGPU code by introducing equation structuring guidelines.
4.) The field output generated by PIConGPU in all E- and B-field components across time and space was directly compared to 2. . (And yes, it agrees :-) . )

I recommend against putting Klaus Steiniger to work now, but I will show him my Mathematica notebooks somewhen in the next few days.