bremsstrahlung splitting

This example illustrates the use of the generic biasing classes to create a bremsstrahlung splitting technique. Note that the EM package also offers a bremsstrahlung splitting, that is built-in to the package.

GB04BOptnBremSplitting The bremsstrahlung splitting is implemented in the GB04BOptnBremSplitting class (BOptn = Biasing Operation), which acts on the final state creation of the bremsstrahlung process.
GB04BOptrBremSplitting Decisions when to apply the GB04BOptnBremSplitting biasing operation are taken by the GB04BOptrBremSplitting (BOptr = Biasing Operator) operator. This one also configures the biasing operation, setting it the splitting factor and its behavior regarding electrons to be biased : only the primary one, or all, and only the first bremsstrahlung operation, or all. These are controled by this specific example commands:
```
/GB04/biasing/setSplittingFactor [N splitting]
/GB04/biasing/biasPrimaryOnly    [true/false]
/GB04/biasing/biasOnlyOnce       [true/false]
```

The geometry is minimal : a single volume to which an instance of GB04BOptrBremSplitting is attached to.

The wrapping of physics processes by G4BiasingProcessInterface processes is simply handled by the G4GenericBiasingPhysics physics constructor, as shown in the main program.