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.
We must remove all definitions of math function in this class and use `PMAcc::algorithms::math` or `util::*` like @PrometheusPi does in the original implementation. (e.g. `util::square`)