Complex-Math: Handle Operations with Scalars

First feature from @BeyondEspresso 's pull #597.

Updated the complex-math library in the radiation-plugin to handle all basic calculations (+,-,*,/) and assignment from scalars.
Both [`std::sqrt`](http://www.cplusplus.com/reference/complex/sqrt/) and [libPMac sqrt](https://github.com/ComputationalRadiationPhysics/picongpu/blob/master/src/libPMacc/include/algorithms/math/defines/sqrt.hpp) support `const`.