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?
@psychocoderHPC You are right, it does not do well with restarts. I'll move it to the end of init() before the field-communication and add a condition to only execute if step==0.

@ax3l You are right, I do this to include the external field in the regular field dump. In my opinion the field output should show the entire field as it exists in the simulation. Keeping external and internal fields separate unnecessarily complicates the use and interpretation of all existing field-plugins. However, it could be interesting to later add an additional feature to dump the external field only,